At the main title screen with ''BUBBLE BOBBLE'' on a cloud, press the following keys:

Code - Effect
Bubble, Jump, Bubble, Jump, Bubble, Jump, Right, F1 key - Bonus Treasure Rooms
Left, Jump, Left, F1 key, Left, Bubble, Left, F1 key - Fast Bubbles
F1 key, Jump, Bubble, Left, Right, Jump, F1 key, Right - Super Mode