diff --git a/sourmash_lib/commands.py b/sourmash_lib/commands.py
index 50e0a12cf1..716040f680 100644
--- a/sourmash_lib/commands.py
+++ b/sourmash_lib/commands.py
@@ -486,6 +486,7 @@ def sbt_index(args):
                         help='signatures to load into SBT')
     parser.add_argument('-k', '--ksize', type=int, default=None)
     parser.add_argument('--traverse-directory', action='store_true')
+    parser.add_argument('--append', action='store_true', default=False)
     parser.add_argument('-x', '--bf-size', type=float, default=1e5)
 
     sourmash_args.add_moltype_args(parser)
@@ -493,8 +494,11 @@ def sbt_index(args):
     args = parser.parse_args(args)
     moltype = sourmash_args.calculate_moltype(args)
 
-    factory = GraphFactory(1, args.bf_size, 4)
-    tree = SBT(factory)
+    if args.append:
+        tree = SBT.load(args.sbt_name, leaf_loader=SigLeaf.load)
+    else:
+        factory = GraphFactory(1, args.bf_size, 4)
+        tree = SBT(factory)
 
     if args.traverse_directory:
         inp_files = list(sourmash_args.traverse_find_sigs(args.signatures))
diff --git a/sourmash_lib/sbt.py b/sourmash_lib/sbt.py
index 08245a4584..75f35b214c 100644
--- a/sourmash_lib/sbt.py
+++ b/sourmash_lib/sbt.py
@@ -156,6 +156,7 @@ def parent(self, pos):
         if pos == 0:
             return None
         p = int(math.floor((pos - 1) / self.d))
+        self.nodes[p] = self.nodes[p].do_load()
         return NodePos(p, self.nodes[p])
 
     def children(self, pos):
@@ -163,6 +164,8 @@ def children(self, pos):
 
     def child(self, parent, pos):
         cd = self.d * parent + pos + 1
+        if self.nodes[cd] is not None:
+            self.nodes[cd] = self.nodes[cd].do_load()
         return NodePos(cd, self.nodes[cd])
 
     def save(self, tag):
@@ -184,6 +187,7 @@ def save(self, tag):
                 structure[i] = None
                 continue
 
+            node = node.do_load()
             basename = os.path.basename(node.name)
             data = {
                 'filename': os.path.join('.sbt.' + basetag,
diff --git a/sourmash_lib/test_sourmash.py b/sourmash_lib/test_sourmash.py
index 524e7b4170..801e94fc5d 100644
--- a/sourmash_lib/test_sourmash.py
+++ b/sourmash_lib/test_sourmash.py
@@ -693,6 +693,52 @@ def test_do_sourmash_sbt_index_traverse():
         assert testdata2 in out
 
 
+def test_do_sourmash_sbt_index_append():
+    with utils.TempDirectory() as location:
+        testdata1 = utils.get_test_data('short.fa')
+        testdata2 = utils.get_test_data('short2.fa')
+        testdata3 = utils.get_test_data('short3.fa')
+        status, out, err = utils.runscript('sourmash',
+                                           ['compute', testdata1, testdata2, testdata3],
+                                           in_directory=location)
+
+        status, out, err = utils.runscript('sourmash',
+                                           ['sbt_index', 'zzz',
+                                            'short.fa.sig', 'short2.fa.sig'],
+                                           in_directory=location)
+
+        assert os.path.exists(os.path.join(location, 'zzz.sbt.json'))
+
+        sbt_name = os.path.join(location, 'zzz',)
+        sig_loc = os.path.join(location, 'short3.fa.sig')
+        status, out, err = utils.runscript('sourmash',
+                                           ['sbt_search', sbt_name, sig_loc])
+        print(out)
+
+        assert testdata1 in out
+        assert testdata2 in out
+        assert testdata3 not in out
+
+        status, out, err = utils.runscript('sourmash',
+                                           ['sbt_index', '--append',
+                                            'zzz',
+                                            'short3.fa.sig'],
+                                           in_directory=location)
+
+        assert os.path.exists(os.path.join(location, 'zzz.sbt.json'))
+
+        sbt_name = os.path.join(location, 'zzz',)
+        sig_loc = os.path.join(location, 'short3.fa.sig')
+        status, out, err = utils.runscript('sourmash',
+                                           ['sbt_search', '--threshold', '0.95',
+                                            sbt_name, sig_loc])
+        print(out)
+
+        assert testdata1 not in out
+        assert testdata2 in out
+        assert testdata3 in out
+
+
 def test_do_sourmash_sbt_search_otherdir():
     with utils.TempDirectory() as location:
         testdata1 = utils.get_test_data('short.fa')