diff --git a/mercurial/filemerge.py b/mercurial/filemerge.py
--- a/mercurial/filemerge.py
+++ b/mercurial/filemerge.py
@@ -290,12 +290,8 @@ def _formatconflictmarker(repo, ctx, tem
     if mark:
         mark = mark.splitlines()[0] # split for safety
 
-    # The <<< marks add 8 to the length, and '...' adds three, so max
-    # length of the actual marker is 69.
-    maxlength = 80 - 8 - 3
-    if len(mark) > maxlength:
-        mark = mark[:maxlength] + '...'
-    return mark
+    # 8 for the prefix of conflict marker lines (e.g. '<<<<<<< ')
+    return util.ellipsis(mark, 80 - 8)
 
 _defaultconflictmarker = ('{node|short} ' +
     '{ifeq(tags, "tip", "", "{tags} ")}' +
diff --git a/tests/test-conflict.t b/tests/test-conflict.t
--- a/tests/test-conflict.t
+++ b/tests/test-conflict.t
@@ -72,9 +72,42 @@ Verify line splitting of custom conflict
   something
   >>>>>>> other: test 1
 
+Verify line trimming of custom conflict marker using multi-byte characters
+
+  $ hg up -q --clean .
+  $ python <<EOF
+  > fp = open('logfile', 'w')
+  > fp.write('12345678901234567890123456789012345678901234567890' +
+  >          '1234567890') # there are 5 more columns for 80 columns
+  > 
+  > # 2 x 4 = 8 columns, but 3 x 4 = 12 bytes
+  > fp.write(u'\u3042\u3044\u3046\u3048'.encode('utf-8'))
+  > 
+  > fp.close()
+  > EOF
+  $ hg add logfile
+  $ hg --encoding utf-8 commit --logfile logfile
+
+  $ cat >> .hg/hgrc <<EOF
+  > [ui]
+  > mergemarkertemplate={desc|firstline}
+  > EOF
+
+  $ hg -q --encoding utf-8 merge 1
+  warning: conflicts during merge.
+  merging a incomplete! (edit conflicts, then use 'hg resolve --mark')
+  [1]
+
+  $ cat a
+  <<<<<<< local: 123456789012345678901234567890123456789012345678901234567890\xe3\x81\x82... (esc)
+  something else
+  =======
+  something
+  >>>>>>> other: branch1
+
 Verify basic conflict markers
 
-  $ hg up -q --clean .
+  $ hg up -q --clean 2
   $ printf "\n[ui]\nmergemarkers=basic\n" >> .hg/hgrc
 
   $ hg merge 1