Battery is listed as "3GS" compatible. After replacing my 3GS battery with this one, the phone went into recovery mode and refused to work properly. A trip to the Apple Store was of no help, only determined the problem through online research and 3rd party software to try to get out of recovery mode. Phone would not work again until original battery is reinstalled. Do the online research - many many other 3GS owners have had battery issues since ios 4.2.1 was released, and this battery is simply not compatible.