diff --git a/datasette/database.py b/datasette/database.py
index fa558045..44467370 100644
--- a/datasette/database.py
+++ b/datasette/database.py
@@ -28,7 +28,7 @@ AttachedDatabase = namedtuple("AttachedDatabase", ("seq", "name", "file"))
 
 class Database:
     def __init__(
-        self, ds, path=None, is_mutable=False, is_memory=False, memory_name=None
+        self, ds, path=None, is_mutable=True, is_memory=False, memory_name=None
     ):
         self.name = None
         self.route = None
@@ -39,7 +39,6 @@ class Database:
         self.memory_name = memory_name
         if memory_name is not None:
             self.is_memory = True
-            self.is_mutable = True
         self.hash = None
         self.cached_size = None
         self._cached_table_counts = None
diff --git a/docs/internals.rst b/docs/internals.rst
index 20797e98..adeec1d8 100644
--- a/docs/internals.rst
+++ b/docs/internals.rst
@@ -426,12 +426,13 @@ The ``db`` parameter should be an instance of the ``datasette.database.Database`
         Database(
             datasette,
             path="path/to/my-new-database.db",
-            is_mutable=True,
         )
     )
 
 This will add a mutable database and serve it at ``/my-new-database``.
 
+Use ``is_mutable=False`` to add an immutable database.
+
 ``.add_database()`` returns the Database instance, with its name set as the ``database.name`` attribute. Any time you are working with a newly added database you should use the return value of ``.add_database()``, for example:
 
 .. code-block:: python
@@ -671,8 +672,8 @@ Instances of the ``Database`` class can be used to execute queries against attac
 
 .. _database_constructor:
 
-Database(ds, path=None, is_mutable=False, is_memory=False, memory_name=None)
-----------------------------------------------------------------------------
+Database(ds, path=None, is_mutable=True, is_memory=False, memory_name=None)
+---------------------------------------------------------------------------
 
 The ``Database()`` constructor can be used by plugins, in conjunction with :ref:`datasette_add_database`, to create and register new databases.
 
@@ -685,7 +686,7 @@ The arguments are as follows:
     Path to a SQLite database file on disk.
 
 ``is_mutable`` - boolean
-    Set this to ``True`` if it is possible that updates will be made to that database - otherwise Datasette will open it in immutable mode and any changes could cause undesired behavior.
+    Set this to ``False`` to cause Datasette to open the file in immutable mode.
 
 ``is_memory`` - boolean
     Use this to create non-shared memory connections.
diff --git a/tests/test_internals_database.py b/tests/test_internals_database.py
index 551f67e1..9e81c1d6 100644
--- a/tests/test_internals_database.py
+++ b/tests/test_internals_database.py
@@ -499,6 +499,7 @@ def test_mtime_ns_is_none_for_memory(app_client):
 
 
 def test_is_mutable(app_client):
+    assert Database(app_client.ds, is_memory=True).is_mutable is True
     assert Database(app_client.ds, is_memory=True, is_mutable=True).is_mutable is True
     assert Database(app_client.ds, is_memory=True, is_mutable=False).is_mutable is False
 
diff --git a/tests/test_internals_datasette.py b/tests/test_internals_datasette.py
index 1dc14cab..249920fe 100644
--- a/tests/test_internals_datasette.py
+++ b/tests/test_internals_datasette.py
@@ -58,7 +58,7 @@ async def test_datasette_constructor():
             "route": "_memory",
             "path": None,
             "size": 0,
-            "is_mutable": False,
+            "is_mutable": True,
             "is_memory": True,
             "hash": None,
         }