Heavy metal's Big Four -- Metallica, Slayer, Megadeth and Anthrax -- will ride together in 2010. In Europe, at least.

After weeks of rumors, the four bands have announced they'll take part in the traveling Sonisphere festival, starting June 16 in Warsaw and continuing June 19 in Prague. More Big Four Sonisphere performances are expected to be announced in the coming weeks; the festival's Web site (sonisphere.com) currently lists 15 dates, including two-night stands in Sofia, Bucharest, Istanbul, and Knebworth, U.K.

There's no word on U.S. dates for the Big Four. Slayer, who co-headlined the 2009 Mayhem Tour, and Megadeth are teaming up for a 26-date American Carnage tour of North America that kicks off January 18 in Seattle. Metallica toured extensively in 2009.

Who would have thought that more than 25 years after its inception, thrash metal's big 4 would not only still be around and more popular than ever, but will now play together for the first time, Metallica drummer Lars Ulrich said in a statement.