diff --git a/myst_nb/convert.py b/myst_nb/convert.py
index 307732b8..f9ffcfe2 100644
--- a/myst_nb/convert.py
+++ b/myst_nb/convert.py
@@ -1,8 +1,13 @@
+"""
+This module contains round-trip conversion between
+myst formatted text documents and notebooks.
+"""
 import json
 from typing import List, Union
 
 from docutils.parsers.rst.directives.misc import TestDirective
 import nbformat as nbf
+import yaml
 
 from mistletoe.base_elements import SourceLines
 from mistletoe.parse_context import ParseContext, get_parse_context, set_parse_context
@@ -12,8 +17,11 @@
 from myst_parser.parse_directives import parse_directive_text
 
 
+DEFAULT_DIRECTIVE = "nb-code"
+
+
 def myst_to_nb(
-    text: Union[str, List[str], SourceLines], directive: str = "nb-cell"
+    text: Union[str, List[str], SourceLines], directive: str = DEFAULT_DIRECTIVE
 ) -> nbf.NotebookNode:
     """Convert text written in the myst format to a notebook.
 
@@ -84,7 +92,8 @@ def myst_to_nb(
                 token = item.node  # type: CodeFence
 
                 # Note: we ignore anything after the directive on the first line
-                # TODO: could log warning about this: ``if token.arguments != ""```
+                # this is reserved for the optional lexer name
+                # TODO: could log warning about if token.arguments != lexer name
 
                 # we use the TestDirective here, since `parse_directive_text`
                 # is setup to skip any option validation for this class
@@ -124,3 +133,61 @@ def myst_to_nb(
         set_parse_context(original_context)
 
     return notebook
+
+
+def from_nbnode(value):
+    """Recursively convert NotebookNode to dict."""
+    if isinstance(value, nbf.NotebookNode):
+        return {k: from_nbnode(v) for k, v in value.items()}
+    return value
+
+
+def nb_to_myst(nb: nbf.NotebookNode, directive: str = DEFAULT_DIRECTIVE):
+    string = ""
+
+    metadata = from_nbnode(nb.metadata)
+    metadata["nbformat"] = nb.nbformat
+    metadata["nbformat_minor"] = nb.nbformat_minor
+
+    # we add the pygments lexer as a directive argument, for use by syntax highlighters
+    pygments_lexer = metadata.get("language_info", {}).get("pygments_lexer", None)
+
+    string += "---\n"
+    string += yaml.safe_dump(metadata)
+    string += "---\n"
+
+    last_cell_md = False
+    for i, cell in enumerate(nb.cells):
+
+        if cell.cell_type == "markdown":
+            metadata = from_nbnode(cell.metadata)
+            if metadata or last_cell_md:
+                if metadata:
+                    string += "\n+++ {}\n".format(json.dumps(metadata))
+                else:
+                    string += "\n+++\n"
+            string += cell.source
+            if not cell.source.endswith("\n"):
+                string += "\n"
+            last_cell_md = True
+
+        elif cell.cell_type == "code":
+            string += "```{{{}}}".format(directive)
+            if pygments_lexer:
+                string += " {}".format(pygments_lexer)
+            string += "\n"
+            metadata = from_nbnode(cell.metadata)
+            if metadata:
+                string += "---\n"
+                string += yaml.safe_dump(metadata)
+                string += "---\n"
+            string += cell.source
+            if not cell.source.endswith("\n"):
+                string += "\n"
+            string += "```\n"
+            last_cell_md = False
+
+        else:
+            raise NotImplementedError("cell {}, type: {}".format(i, cell.cell_type))
+
+    return string.rstrip() + "\n"
diff --git a/tests/test_convert/test_basic.ipynb b/tests/roundtrip/basic.ipynb
similarity index 88%
rename from tests/test_convert/test_basic.ipynb
rename to tests/roundtrip/basic.ipynb
index ddf955a6..c8888b87 100644
--- a/tests/test_convert/test_basic.ipynb
+++ b/tests/roundtrip/basic.ipynb
@@ -47,6 +47,10 @@
   }
  ],
  "metadata": {
+  "language_info": {
+   "name": "python",
+   "pygments_lexer": "ipython3"
+  },
   "orphan": true
  },
  "nbformat": 4,
diff --git a/tests/roundtrip/basic.mystnb b/tests/roundtrip/basic.mystnb
new file mode 100644
index 00000000..347fe733
--- /dev/null
+++ b/tests/roundtrip/basic.mystnb
@@ -0,0 +1,27 @@
+---
+language_info:
+  name: python
+  pygments_lexer: ipython3
+nbformat: 4
+nbformat_minor: 4
+orphan: true
+---
+
+a
+
+b
+c
+```{nb-code} ipython3
+---
+tags:
+- hide-code
+---
+a = 1
+print(a)
+```
+
+c
+
++++ {"tags": ["hide-cell"]}
+
+d
diff --git a/tests/test_convert.py b/tests/test_convert.py
index 9f98572f..18513caf 100644
--- a/tests/test_convert.py
+++ b/tests/test_convert.py
@@ -1,34 +1,20 @@
-from textwrap import dedent
-import nbformat
-from myst_nb.convert import myst_to_nb
-
-
-def test_basic(file_regression):
-    text = dedent(
-        """\
-        ---
-        orphan: true
-        ---
+from pathlib import Path
 
-        a
+import nbformat
+from myst_nb.convert import myst_to_nb, nb_to_myst
 
-        b
-        c
-        ```{nb-cell}
-        ---
-        tags: ["hide-code"]
-        ---
-        a = 1
-        print(a)
-        ```
+SOURCEDIR = Path(__file__).parent.joinpath("roundtrip")
 
-        c
 
-        +++ {"tags": ["hide-cell"]}
+def test_myst_to_nb(file_regression):
+    text = SOURCEDIR.joinpath("basic.mystnb").read_text()
+    notebook = myst_to_nb(text, directive="nb-code")
+    file_regression.check(
+        nbformat.writes(notebook), fullpath=SOURCEDIR.joinpath("basic.ipynb")
+    )
 
-        d
 
-        """
-    )
-    notebook = myst_to_nb(text, directive="nb-cell")
-    file_regression.check(nbformat.writes(notebook), extension=".ipynb")
+def test_nb_to_myst(file_regression):
+    text = SOURCEDIR.joinpath("basic.ipynb").read_text()
+    output = nb_to_myst(nbformat.reads(text, 4), directive="nb-code")
+    file_regression.check(output, fullpath=SOURCEDIR.joinpath("basic.mystnb"))