The following are known Action Replay Codes for Stuart Little 2 on Nintendo Game Boy Advance (GBA).
These codes were also known as GameShark Codes and in most cases will work on both devices.

These codes are for the U.S. version of the game.

Master Code – Must Be On
4e00d100
b0edee03
de6b3e0d
940de7a5

Start With 99 Lives
a2d8789e
9c4af1dc