Bones of mammals exhibit progressive stages of weathering during their time of subaerial exposure. Consequently, the study of bone weathering in fossil assemblages may help to assess the period represented by an accumulation of bones. Stages of bone decomposition due to subaerial weathering have been identified in assemblages of fossil macromammals from Olduvai Gorge, Tanzania. A modern bone assemblage collected by spotted hyenas is used to devise a method for recognizing attritional accumulations of bones from weathering characteristics. This method, which involves study of long bone diaphyses, is applied to Plio-Pleistocene faunal assemblages from Olduvai, 1.70–1.85 ma old. Previous work indicates that early hominids had an important role in the collection of fauna at five of the six sites studied. It is shown that animal bones were accumulated at each site over a period of probably 5–10 yr or more. The length of this period, along with other taphonomic evidence, suggests that the processes of bone aggregation at these sites differed from those at the short-term campsites of modern, tropical hunter-gatherers.