Spotify is the fastest growing music-streaming app on the iPhone in the U.S., new data show, highlighting why it is particularly vulnerable to Apple’s relaunch of rival Beats Music this summer.

Prepared by research firm 7ParkData, the data look at the relative strengths of six key music-streaming apps across Apple and Android mobile devices in the U.S. (where the two platforms run neck and neck in terms of market share) in the six months ending in mid-April.