Until now, scientists had seen gorillas use tools only in captivity. Among the great apes, tool use in the wild was thought to be a survival skill reserved for smaller chimpanzees and orangutans.

The research in the Republic of Congo's rainforests was led by Thomas Breuer of the Wildlife Conservation Society at the Bronx Zoo, which released details of his study. Breuer is in Africa and was not immediately available for an interview.

''This is a truly astounding discovery,'' he said in a statement. ''Tool usage in wild apes provides us with valuable insights into the evolution of our own species and the abilities of other species.''

Other scientists said the observations were important, but not surprising.

Breuer's observations were made late last year in a marshy clearing called Mbeli Baia located in Nouabalé-Ndoki National Park where monitoring has been ongoing since February 1995.

The first instance was observed last October when a female gorilla, nicknamed Leah by scientists, attempted to wade through a pool of water created by elephants, but found herself waist deep after only a few steps. Climbing out of the pool, she retrieved a branch from a dead tree and used the stick to test the depth of the water.

In November, a second female gorilla, named Efi, used a detached tree trunk to support herself with one hand while digging for herbs with the other hand. She also used the tree trunk as a bridge to cross a muddy patch of ground.