diff --git a/bwamem.c b/bwamem.c
index 84c84241..42788c49 100644
--- a/bwamem.c
+++ b/bwamem.c
@@ -43,7 +43,7 @@ mem_opt_t *mem_opt_init()
 	mem_opt_t *o;
 	o = calloc(1, sizeof(mem_opt_t));
 	o->flag = 0;
-	o->a = 1; o->b = 4; o->q = 6; o->r = 1; o->w = 100;
+	o->a = 1; o->b = 4; o->q = 6; o->r = 1; o->w = 60; o->max_w = 500;
 	o->pen_unpaired = 9;
 	o->pen_clip = 5;
 	o->min_seed_len = 19;
@@ -492,7 +492,7 @@ static inline int cal_max_gap(const mem_opt_t *opt, int qlen)
 
 void mem_chain2aln(const mem_opt_t *opt, int64_t l_pac, const uint8_t *pac, int l_query, const uint8_t *query, const mem_chain_t *c, mem_alnreg_v *av)
 {
-	int i, k;
+	int i, k, max_off[2], aw[2]; // aw: actual bandwidth used in extension
 	int64_t rlen, rmax[2], tmp, max = 0;
 	const mem_seed_t *s;
 	uint8_t *rseq = 0;
@@ -549,16 +549,22 @@ void mem_chain2aln(const mem_opt_t *opt, int64_t l_pac, const uint8_t *pac, int
 
 		a = kv_pushp(mem_alnreg_t, *av);
 		memset(a, 0, sizeof(mem_alnreg_t));
+		a->w = aw[0] = aw[1] = opt->w;
 
 		if (s->qbeg) { // left extension
 			uint8_t *rs, *qs;
-			int qle, tle, gtle, gscore;
+			int qle, tle, gtle, gscore, tmps = -1;
 			qs = malloc(s->qbeg);
 			for (i = 0; i < s->qbeg; ++i) qs[i] = query[s->qbeg - 1 - i];
 			tmp = s->rbeg - rmax[0];
 			rs = malloc(tmp);
 			for (i = 0; i < tmp; ++i) rs[i] = rseq[tmp - 1 - i];
-			a->score = ksw_extend(s->qbeg, qs, tmp, rs, 5, opt->mat, opt->q, opt->r, opt->w, s->len * opt->a, &qle, &tle, &gtle, &gscore);
+			for (aw[0] = opt->w;; aw[0] <<= 1) {
+				a->score = ksw_extend(s->qbeg, qs, tmp, rs, 5, opt->mat, opt->q, opt->r, aw[0], s->len * opt->a, &qle, &tle, &gtle, &gscore, &max_off[0]);
+				if (bwa_verbose >= 4) printf("L\t%d < %d; w=%d; max_off=%d\n", tmps, a->score, aw[0], max_off[0]); fflush(stdout);
+				if (a->score == tmps || aw[0]<<1 > opt->max_w || max_off[0] < (aw[0]>>1) + (aw[0]>>2)) break;
+				tmps = a->score;
+			}
 			// check whether we prefer to reach the end of the query
 			if (gscore <= 0 || gscore <= a->score - opt->pen_clip) a->qb = s->qbeg - qle, a->rb = s->rbeg - tle; // local hits
 			else a->qb = 0, a->rb = s->rbeg - gtle; // reach the end
@@ -566,16 +572,21 @@ void mem_chain2aln(const mem_opt_t *opt, int64_t l_pac, const uint8_t *pac, int
 		} else a->score = s->len * opt->a, a->qb = 0, a->rb = s->rbeg;
 
 		if (s->qbeg + s->len != l_query) { // right extension
-			int qle, tle, qe, re, gtle, gscore;
+			int qle, tle, qe, re, gtle, gscore, tmps = -1;
 			qe = s->qbeg + s->len;
 			re = s->rbeg + s->len - rmax[0];
 			assert(re >= 0);
-			a->score = ksw_extend(l_query - qe, query + qe, rmax[1] - rmax[0] - re, rseq + re, 5, opt->mat, opt->q, opt->r, opt->w, a->score, &qle, &tle, &gtle, &gscore);
+			for (aw[1] = opt->w;; aw[1] <<= 1) {
+				a->score = ksw_extend(l_query - qe, query + qe, rmax[1] - rmax[0] - re, rseq + re, 5, opt->mat, opt->q, opt->r, aw[1], a->score, &qle, &tle, &gtle, &gscore, &max_off[1]);
+				if (bwa_verbose >= 4) printf("R\t%d < %d; w=%d; max_off=%d\n", tmps, a->score, aw[1], max_off[1]); fflush(stdout);
+				if (a->score == tmps || aw[1]<<1 > opt->max_w || max_off[1] < (aw[1]>>1) + (aw[1]>>2)) break;
+				tmps = a->score;
+			}
 			// similar to the above
 			if (gscore <= 0 || gscore <= a->score - opt->pen_clip) a->qe = qe + qle, a->re = rmax[0] + re + tle;
 			else a->qe = l_query, a->re = rmax[0] + re + gtle;
 		} else a->qe = l_query, a->re = s->rbeg + s->len;
-		if (bwa_verbose >= 4) { printf("[%d] score=%d\t[%d,%d) <=> [%ld,%ld)\n", k, a->score, a->qb, a->qe, (long)a->rb, (long)a->re); fflush(stdout); }
+		if (bwa_verbose >= 4) { printf("[%d]\taw={%d,%d}\tscore=%d\t[%d,%d) <=> [%ld,%ld)\n", k, aw[0], aw[1], a->score, a->qb, a->qe, (long)a->rb, (long)a->re); fflush(stdout); }
 
 		// compute seedcov
 		for (i = 0, a->seedcov = 0; i < c->n; ++i) {
@@ -583,6 +594,7 @@ void mem_chain2aln(const mem_opt_t *opt, int64_t l_pac, const uint8_t *pac, int
 			if (t->qbeg >= a->qb && t->qbeg + t->len <= a->qe && t->rbeg >= a->rb && t->rbeg + t->len <= a->re) // seed fully contained
 				a->seedcov += t->len; // this is not very accurate, but for approx. mapQ, this is good enough
 		}
+		a->w = aw[0] > aw[1]? aw[0] : aw[1];
 	}
 	free(srt); free(rseq);
 }
@@ -750,7 +762,7 @@ void mem_sam_se(const mem_opt_t *opt, const bntseq_t *bns, const uint8_t *pac, b
 			h.qual = p->secondary >= 0? 0 : mem_approx_mapq_se(opt, p);
 			if (k == 0) mapq0 = h.qual;
 			else if (h.qual > mapq0) h.qual = mapq0;
-			bwa_hit2sam(&str, opt->mat, opt->q, opt->r, opt->w, bns, pac, s, &h, opt->flag&MEM_F_HARDCLIP, m);
+			bwa_hit2sam(&str, opt->mat, opt->q, opt->r, p->w, bns, pac, s, &h, opt->flag&MEM_F_HARDCLIP, m);
 		}
 	} else bwa_hit2sam(&str, opt->mat, opt->q, opt->r, opt->w, bns, pac, s, 0, opt->flag&MEM_F_HARDCLIP, m);
 	s->sam = str.s;
diff --git a/bwamem.h b/bwamem.h
index 3f4ce322..4d632e74 100644
--- a/bwamem.h
+++ b/bwamem.h
@@ -22,6 +22,7 @@ typedef struct {
 	int pen_unpaired;       // phred-scaled penalty for unpaired reads
 	int pen_clip;           // clipping penalty. This score is not deducted from the DP score.
 	int w;                  // band width
+	int max_w;              // max band width
 
 	int flag;               // see MEM_F_* macros
 	int min_seed_len;       // minimum seed length
@@ -45,6 +46,7 @@ typedef struct {
 	int sub;        // 2nd best SW score
 	int csub;       // SW score of a tandem hit
 	int sub_n;      // approximate number of suboptimal hits
+	int w;          // actual band width used in extension
 	int seedcov;    // length of regions coverged by seeds
 	int secondary;  // index of the parent hit shadowing the current hit; <0 if primary
 } mem_alnreg_t;
diff --git a/ksw.c b/ksw.c
index b97fed5d..3747cb04 100644
--- a/ksw.c
+++ b/ksw.c
@@ -359,11 +359,11 @@ typedef struct {
 	int32_t h, e;
 } eh_t;
 
-int ksw_extend(int qlen, const uint8_t *query, int tlen, const uint8_t *target, int m, const int8_t *mat, int gapo, int gape, int w, int h0, int *_qle, int *_tle, int *_gtle, int *_gscore)
+int ksw_extend(int qlen, const uint8_t *query, int tlen, const uint8_t *target, int m, const int8_t *mat, int gapo, int gape, int w, int h0, int *_qle, int *_tle, int *_gtle, int *_gscore, int *_max_off)
 {
 	eh_t *eh; // score array
 	int8_t *qp; // query profile
-	int i, j, k, gapoe = gapo + gape, beg, end, max, max_i, max_j, max_gap, max_ie, gscore;
+	int i, j, k, gapoe = gapo + gape, beg, end, max, max_i, max_j, max_gap, max_ie, gscore, max_off;
 	if (h0 < 0) h0 = 0;
 	// allocate memory
 	qp = malloc(qlen * m);
@@ -386,6 +386,7 @@ int ksw_extend(int qlen, const uint8_t *query, int tlen, const uint8_t *target,
 	w = w < max_gap? w : max_gap;
 	// DP loop
 	max = h0, max_i = max_j = -1; max_ie = -1, gscore = -1;
+	max_off = 0;
 	beg = 0, end = qlen;
 	for (i = 0; LIKELY(i < tlen); ++i) {
 		int f = 0, h1, m = 0, mj = -1;
@@ -410,7 +411,7 @@ int ksw_extend(int qlen, const uint8_t *query, int tlen, const uint8_t *target,
 			h = h > e? h : e;
 			h = h > f? h : f;
 			h1 = h;             // save H(i,j) to h1 for the next column
-			mj = m > h? mj : j;
+			mj = m > h? mj : j; // record the position where max score is achieved
 			m = m > h? m : h;   // m is stored at eh[mj+1]
 			h -= gapoe;
 			h = h > 0? h : 0;
@@ -426,7 +427,10 @@ int ksw_extend(int qlen, const uint8_t *query, int tlen, const uint8_t *target,
 			gscore = gscore > h1? gscore : h1;
 		}
 		if (m == 0) break;
-		if (m > max) max = m, max_i = i, max_j = mj;
+		if (m > max) {
+			max = m, max_i = i, max_j = mj;
+			max_off = max_off > abs(mj - i)? max_off : abs(mj - i);
+		}
 		// update beg and end for the next round
 		for (j = mj; j >= beg && eh[j].h; --j);
 		beg = j + 1;
@@ -439,6 +443,7 @@ int ksw_extend(int qlen, const uint8_t *query, int tlen, const uint8_t *target,
 	if (_tle) *_tle = max_i + 1;
 	if (_gtle) *_gtle = max_ie + 1;
 	if (_gscore) *_gscore = gscore;
+	if (_max_off) *_max_off = max_off;
 	return max;
 }
 
diff --git a/ksw.h b/ksw.h
index d2975dec..6d1f7cfa 100644
--- a/ksw.h
+++ b/ksw.h
@@ -102,7 +102,7 @@ extern "C" {
 	 *
 	 * @return        best semi-local alignment score
 	 */
-	int ksw_extend(int qlen, const uint8_t *query, int tlen, const uint8_t *target, int m, const int8_t *mat, int gapo, int gape, int w, int h0, int *qle, int *tle, int *gtle, int *gscore);
+	int ksw_extend(int qlen, const uint8_t *query, int tlen, const uint8_t *target, int m, const int8_t *mat, int gapo, int gape, int w, int h0, int *qle, int *tle, int *gtle, int *gscore, int *max_off);
 
 #ifdef __cplusplus
 }