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A route census was carried out by one observer using the same methods along the same survey routes during the same seasons in 1996 and 2000 to estimate the abundance of medium- and large-sized diurnal mammals within the home range of the chimpanzees of M Group in the Mahale Mountains, western Tanzania. All eight mammalian species censused were known to have been consumed by chimpanzees, and there are other resident predators such as leopards and crowned hawk eagles. No statistically signifi cant differences were found in group densities of gregarious species or individual densities of non-gregarious species between the two data sets. However, frequencies of encounter with and estimates of group size of some species suggested possible decrease (bushbucks) or increase/non-decrease (red-tailed monkeys, yellow baboons, red colobus, warthogs, blue duikers and forest squirrels) from 1996 to 2000, although such an assumption could not be made for blue monkeys due to their low densities in both years. No quantitative predation data for the relevant period exist, but the results show that predation pressure as a whole including hunts by chimpanzees did not seem to exceed population growth rates of the prey mammals, with the exclusion of blue monkeys and the possible exception of bushbucks, during the 1996-2000 period at Mahale.