Of all Internet pages that were downloaded to mobile devices globally in May, 24.6 percent were downloaded through Opera's browser and 22.3 percent via iPhone, StatCounter said.
The top spot has see-sawed this year.
'Opera began the year in the number one slot but iPhone overtook it in February,' Aodhan Cullen, chief executive of StatCounter, said in a statement. 'It will be fascinating to watch how this battle plays out over the year.'
Opera sells its browser to many cellphone makers and operators, and consumers can directly download it for free, while the Apple browser's ranking reflects only iPhone users surfing the Internet.
Nokia, the world's top cellphone maker, retained third place in mobile browsers with 17.9 percent of the market, StatCounter said.
StatCounter said its research data is based on four billion pages loaded per month.
(Reporting by Tarmo Virki; Editing by Gary Hill) Keywords: MOBILE BROWSERS/ (firstname.lastname@example.org, +358-9-680 50 235, Reuters messaging: email@example.com) COPYRIGHT Copyright Thomson Reuters 2009. All rights reserved. The copying, republication or redistribution of Reuters News Content, including by framing or similar means, is expressly prohibited without the prior written consent of Thomson Reuters.