diff --git a/swh/loader/mercurial/bundle20_reader.py b/swh/loader/mercurial/bundle20_reader.py
--- a/swh/loader/mercurial/bundle20_reader.py
+++ b/swh/loader/mercurial/bundle20_reader.py
@@ -323,7 +323,7 @@
         commit['time'] = datetime.fromtimestamp(float(tstamp))
         commit['time_offset_seconds'] = int(tz)
         if extra:
-            commit['extra'] = extra[0]
+            commit['extra'] = b' '.join(extra)
         commit['changed_files'] = parts[3:]
         return commit
 
diff --git a/swh/loader/mercurial/loader.py b/swh/loader/mercurial/loader.py
--- a/swh/loader/mercurial/loader.py
+++ b/swh/loader/mercurial/loader.py
@@ -376,6 +376,10 @@
                 for e in extra.split(b'\x00'):
                     k, v = e.split(b':', 1)
                     k = k.decode('utf-8')
+                    # transplant_source stores binary reference to a changeset
+                    # prefer to dump hexadecimal one in the revision metadata
+                    if k == 'transplant_source':
+                        v = hash_to_hex(v)
                     extra_meta.append([k, v])
 
             revision = {
diff --git a/swh/loader/mercurial/tests/resources/transplant.tgz b/swh/loader/mercurial/tests/resources/transplant.tgz
new file mode 100644
index 0000000000000000000000000000000000000000..0000000000000000000000000000000000000000
GIT binary patch
literal 0
Hc$@<O00001

literal 0
Hc$@<O00001

diff --git a/swh/loader/mercurial/tests/test_loader.py b/swh/loader/mercurial/tests/test_loader.py
--- a/swh/loader/mercurial/tests/test_loader.py
+++ b/swh/loader/mercurial/tests/test_loader.py
@@ -29,7 +29,7 @@
         self.storage = self.loader.storage
 
 
-class LoaderTestT1(BaseHgLoaderTest):
+class WithoutReleaseLoaderTest(BaseHgLoaderTest):
     """Load a mercurial repository without release
 
     """
@@ -190,7 +190,7 @@
         self.assertEqual(self.loader.visit_status(), 'full')
 
 
-class LoaderTest2(BaseHgLoaderTest, CommonHgLoaderData):
+class WithReleaseLoaderTest(BaseHgLoaderTest, CommonHgLoaderData):
     """Load a mercurial repository with release
 
     """
@@ -247,3 +247,43 @@
         self.assertCountRevisions(0)
         self.assertCountReleases(0)
         self.assertCountSnapshots(0)
+
+
+class WithTransplantLoaderTest(BaseHgLoaderTest):
+    """Load a mercurial repository where transplant operations
+    have been used.
+
+    """
+    def setUp(self):
+        super().setUp(archive_name='transplant.tgz', filename='transplant')
+
+    def test_load(self):
+        # load hg repository
+        self.loader.load(
+            origin_url=self.repo_url,
+            visit_date='2019-05-23 12:06:00+00',
+            directory=self.destination_path)
+
+        # collect swh revisions
+        origin_id = self.storage.origin_get([
+            {'type': 'hg', 'url': self.repo_url}])[0]['id']
+        snapshot = self.storage.snapshot_get_latest(origin_id)
+        revisions = []
+        for branch in snapshot['branches'].values():
+            if branch['target_type'] != 'revision':
+                continue
+            revisions.append(branch['target'])
+
+        # extract original changesets info and the transplant sources
+        hg_changesets = set()
+        transplant_sources = set()
+        for rev in self.storage.revision_log(revisions):
+            hg_changesets.add(rev['metadata']['node'])
+            for k, v in rev['metadata']['extra_headers']:
+                if k == 'transplant_source':
+                    transplant_sources.add(v)
+
+        # check extracted data are valid
+        self.assertTrue(len(hg_changesets) > 0)
+        self.assertTrue(len(transplant_sources) > 0)
+        self.assertTrue(transplant_sources.issubset(hg_changesets))