diff --git a/imap-codec/src/codec/encode.rs b/imap-codec/src/codec/encode.rs
index 1e045e82..c4f15ffc 100644
--- a/imap-codec/src/codec/encode.rs
+++ b/imap-codec/src/codec/encode.rs
@@ -45,7 +45,7 @@
 //! C: Pa²²W0rD
 //! ```
 
-use std::{borrow::Borrow, io::Write, num::NonZeroU32};
+use std::{borrow::Borrow, io::Write, num::{NonZeroU32, NonZeroU64}};
 
 use base64::{engine::general_purpose::STANDARD as base64, Engine};
 use chrono::{DateTime as ChronoDateTime, FixedOffset};
@@ -980,6 +980,7 @@ impl<'a> EncodeIntoContext for MessageDataItemName<'a> {
             Self::Rfc822Size => ctx.write_all(b"RFC822.SIZE"),
             Self::Rfc822Text => ctx.write_all(b"RFC822.TEXT"),
             Self::Uid => ctx.write_all(b"UID"),
+            Self::ModSeq => ctx.write_all(b"MODSEQ"),
         }
     }
 }
@@ -1044,6 +1045,12 @@ impl EncodeIntoContext for NonZeroU32 {
     }
 }
 
+impl EncodeIntoContext for NonZeroU64 {
+    fn encode_ctx(&self, ctx: &mut EncodeContext) -> std::io::Result<()> {
+        write!(ctx, "{self}")
+    }
+}
+
 impl<'a> EncodeIntoContext for Capability<'a> {
     fn encode_ctx(&self, ctx: &mut EncodeContext) -> std::io::Result<()> {
         write!(ctx, "{}", self)
@@ -1295,6 +1302,7 @@ impl<'a> EncodeIntoContext for Data<'a> {
                     root.encode_ctx(ctx)?;
                 }
             }
+
             #[cfg(feature = "ext_id")]
             Data::Id { parameters } => {
                 ctx.write_all(b"* ID ")?;
@@ -1448,6 +1456,11 @@ impl<'a> EncodeIntoContext for MessageDataItem<'a> {
                 nstring.encode_ctx(ctx)
             }
             Self::Uid(uid) => write!(ctx, "UID {uid}"),
+            Self::ModSeq(modseq) => {
+                ctx.write_all(b"MODSEQ (")?;
+                modseq.encode_ctx(ctx)?;
+                ctx.write_all(b")")
+            }
         }
     }
 }
diff --git a/imap-codec/src/core.rs b/imap-codec/src/core.rs
index 90a810b7..664fae41 100644
--- a/imap-codec/src/core.rs
+++ b/imap-codec/src/core.rs
@@ -1,4 +1,4 @@
-use std::{borrow::Cow, num::NonZeroU32, str::from_utf8};
+use std::{borrow::Cow, num::{NonZeroU32, NonZeroU64}, str::from_utf8};
 
 #[cfg(not(feature = "quirk_crlf_relaxed"))]
 use abnf_core::streaming::crlf;
@@ -65,6 +65,10 @@ pub(crate) fn nz_number(input: &[u8]) -> IMAPResult<&[u8], NonZeroU32> {
     map_res(number, NonZeroU32::try_from)(input)
 }
 
+pub(crate) fn nz_number64(input: &[u8]) -> IMAPResult<&[u8], NonZeroU64> {
+    map_res(number64, NonZeroU64::try_from)(input)
+}
+
 // ----- string -----
 
 /// `string = quoted / literal`
diff --git a/imap-codec/src/fetch.rs b/imap-codec/src/fetch.rs
index ec536bce..f51c2cd9 100644
--- a/imap-codec/src/fetch.rs
+++ b/imap-codec/src/fetch.rs
@@ -1,4 +1,4 @@
-use std::num::NonZeroU32;
+use std::num::{NonZeroU32};
 
 use abnf_core::streaming::sp;
 use imap_types::{
@@ -15,7 +15,7 @@ use nom::{
 
 use crate::{
     body::body,
-    core::{astring, nstring, number, nz_number},
+    core::{astring, nstring, number, nz_number64, nz_number},
     datetime::date_time,
     decode::IMAPResult,
     envelope::envelope,
@@ -83,6 +83,7 @@ pub(crate) fn fetch_att(input: &[u8]) -> IMAPResult<&[u8], MessageDataItemName>
         value(MessageDataItemName::Rfc822Size, tag_no_case(b"RFC822.SIZE")),
         value(MessageDataItemName::Rfc822Text, tag_no_case(b"RFC822.TEXT")),
         value(MessageDataItemName::Rfc822, tag_no_case(b"RFC822")),
+        value(MessageDataItemName::ModSeq, tag_no_case(b"MODSEQ")),
     ))(input)
 }
 
@@ -175,6 +176,9 @@ pub(crate) fn msg_att_static(input: &[u8]) -> IMAPResult<&[u8], MessageDataItem>
         map(tuple((tag_no_case(b"UID"), sp, uniqueid)), |(_, _, uid)| {
             MessageDataItem::Uid(uid)
         }),
+        map(tuple((tag_no_case(b"MODSEQ "), delimited(tag("("), nz_number64, tag(")")))), |(_, modseq)| {
+            MessageDataItem::ModSeq(modseq)
+        }),
     ))(input)
 }
 
diff --git a/imap-types/src/fetch.rs b/imap-types/src/fetch.rs
index b20f6486..40f849c0 100644
--- a/imap-types/src/fetch.rs
+++ b/imap-types/src/fetch.rs
@@ -2,7 +2,7 @@
 
 use std::{
     fmt::{Display, Formatter},
-    num::NonZeroU32,
+    num::{NonZeroU32, NonZeroU64},
 };
 
 #[cfg(feature = "arbitrary")]
@@ -229,6 +229,9 @@ pub enum MessageDataItemName<'a> {
     /// UID
     /// ```
     Uid,
+
+    /// The ModSeq of CONDSTORE
+    ModSeq,
 }
 
 /// Message data item.
@@ -356,6 +359,9 @@ pub enum MessageDataItem<'a> {
     /// UID
     /// ```
     Uid(NonZeroU32),
+
+    /// The ModSeq value described in CONDSTORE
+    ModSeq(NonZeroU64),
 }
 
 /// A part specifier is either a part number or one of the following: