An Australian man descending a rocky slope in the United States as part of a 650km adventure race was struck and killed by a falling boulder, police said.

Eight members of Nigel Aylott's team and an American team were using ropes to lower themselves when a boulder that weighed more than 135kg came loose, Chief Deputy Sheriff Will Reichardt said.

Aylott, who was not wearing a helmet, was struck in the head. Two other racers were injured.

The Australian and American teams were leading the Subaru Primal Quest race, which involved hiking, biking and kayaking stages. Helmets were required for only the biking portion.

The race, which began on Sunday and was expected to last about 10 days, was halted after Aylott's death.

Aylott, 38, was an experienced racer, finishing in the top three in 12 adventure races around the world, according to his team's website.

A personal profile of Mr Aylott posted on an adventure racing website said he was best known for his running and long distance cross country navigation, known as rogaining.

The profile said he boasted numerous Australian long distance running records and titles.

He twice won the Australian Rogaining Championships, and was a former world rogaining champion.

His successful adventure-racing career included top three finishes in last year's Primal Quest race in California, the Eco-Challenge expedition race in Borneo in 2000, and the Southern Traverse race in New Zealand in 1999.

Mr Aylott was a business analyst for Telstra, the profile said.

© 2004 AP