diff --git a/bumble/gatt_client.py b/bumble/gatt_client.py
index 76564245..23485df8 100644
--- a/bumble/gatt_client.py
+++ b/bumble/gatt_client.py
@@ -288,8 +288,6 @@ class Client:
             self._bearer_id = (
                 f'[0x{bearer.connection.handle:04X}|CID=0x{bearer.source_cid:04X}]'
             )
-            # Fill the mtu.
-            bearer.on_att_mtu_update(att.ATT_DEFAULT_MTU)
             self.connection = bearer.connection
         else:
             bearer.on(bearer.EVENT_DISCONNECTION, self.on_disconnection)
diff --git a/bumble/gatt_server.py b/bumble/gatt_server.py
index 5ab94303..3fbbe2fd 100644
--- a/bumble/gatt_server.py
+++ b/bumble/gatt_server.py
@@ -115,7 +115,6 @@ class Server(utils.EventEmitter):
                 channel.connection.handle,
                 channel.source_cid,
             )
-            channel.att_mtu = att.ATT_DEFAULT_MTU
             channel.sink = lambda pdu: self.on_gatt_pdu(
                 channel, att.ATT_PDU.from_bytes(pdu)
             )
diff --git a/bumble/l2cap.py b/bumble/l2cap.py
index 7023a870..f01276e9 100644
--- a/bumble/l2cap.py
+++ b/bumble/l2cap.py
@@ -1647,7 +1647,9 @@ class LeCreditBasedChannel(utils.EventEmitter):
         self.connection_result = None
         self.disconnection_result = None
         self.drained = asyncio.Event()
-        self.att_mtu = 0  # Filled by GATT client or server later.
+        # Core Specification Vol 3, Part G, 5.3.1 ATT_MTU
+        # ATT_MTU shall be set to the minimum of the MTU field values of the two devices.
+        self.att_mtu = min(mtu, peer_mtu)
 
         self.drained.set()