IPhone app developer Tapulous says its sales have approached $1 million a month, providing fresh evidence of the growing success of start-ups designing programs for Apple Inc's mobile device.

Tapulous -- with a mere 20 employees -- said its Tap Tap Revenge game series has now been installed more than 20 million times, with more than 600 million total games played.

Earlier this year, research group comScore said the game had been installed by one-third of Apple app users.

The company says it's profitable, but it won't reveal precise sales data. Tapulous' chief executive said he expects it to ride a wave of exponential growth in mobile app commerce in the next two years, similar to that seen recently by social gaming companies like Zynga, Playfish and Playdom.

Playfish was recently acquired by Electronic Arts for $275 million in cash.

It's going to be big and all of a sudden people are going to say, 'holy cow, where did those guys come from?' said CEO Bart Decrem.

The install base for app developers is growing fast. Apple has sold more than 50 million iPhones and iPod touches, which use the same software.

Tapulous raised $2.8 million from investors, which include such notables as Salesforce.com CEO Marc Benioff and venture capital firm Khosla Ventures.

Tapulous generates sales by charging for some games, through ads, and by selling songs through its games.

(Reporting by Gabriel Madway; Editing by Gary Hill)