Google's Android continued as the top smartphone operating system in the U.S. in ComScore's latest ranking, while Apple's iPhone displaced the BlackBerry for second place.

Android and iPhone devices both gained in popularity in the ComScore survey of 30,000 U.S. mobile phone subscribers. Meanwhile, the BlackBerry from Research in Motion declined by nearly 5 percentage points, dropping to 25.7% of the market for the three months ending April 30.

Android had 36.4% of the smartphone market at the end of April, a 13% improvement and a 5.2 percentage-point gain over the three months ending Jan. 31. Android hit the top ranking for the first time in that January survey, which was reported in March .

Apple 's iPhone had 26% of the smartphone market in the latest report, an increase of 5% and a 1.3 percentage point gain over the previous report.

Along with RIM, Microsoft Windows Phone and Windows Mobile operating systems also declined, although on a smaller base. Microsoft's OS went from 8% of smartphones at the end of January to 6.7% at the end of April. Hewlett-Packard's Palm Web OS dropped from 3.2% to 2.6% over the same period, ComScore reported.

Android's dominance had been predicted, partly because the OS runs on devices from several major phone manufacturers, while only Apple and RIM make iPhone and BlackBerry devices,respectively.

The news of RIM's 12% decline in U.S. smartphones is especially important, because the loss comes at a time when overall smartphone usage increased by 13% for the quarter ending in April. RIM has discussed a slowdown in smartphone rollouts amid financial performance problems in recent weeks as its first quarter-earnings report approaches on June 16.

Meanwhile, Apple is expected to announce its iOS 5 today , promising further improvements to the popular smartphone OS, although news of fifth-generation iPhone hardware is expected at a later date.