The impacts of ungulates on small mammals in an East African savanna habitat were investigated by monitoring the population and community responses of small mammals on replicated 4-ha plots from which ungulates had been excluded. The dominant small mammal in this habitat is the pouched mouse, Saccostomusmearnsi, a medium-sized murid rodent. Eight other small mammal species, including Arvicanthis sp., Mus sp., Mastomys sp., Dendromus sp., Crocidura sp., and, rarely, Tatera sp., Aethomys sp., and Acomys sp., were also captured. The dominant ungulates are elephant (Loxodonta africana), giraffe (Giraffa camelopardalis), Grevy's and common zebra (Equus grevyi and E. burchelli), buffalo (Syncerus cafer), eland (Taurotragus oryx), Grant's gazelle (Gazella granti), and domestic cattle. Within 1 year, S. mearnsi populations had responded dramatically to the exclusion of large mammals by a two-fold increase in density, a difference that was maintained through pronounced seasonal fluctuations in the second year. Though individual pouched mice showed no significant differences in their use of space with and without ungulates, male S. mearnsi maintained significantly higher body weights in the absence of ungulates, indicating that habitat quality had increased. One other species, Mastomys sp., also increased in the absence of ungulates. Overall, the small mammal community maintained relatively constant species diversity on the plots to which ungulates did not have access. On the plots to which ungulates did have access, on the other hand, there was a rapid 75% decrease in diversity in the control plots during one trapping session. Ungulates are most likely affecting small mammals through their effects on food quality, since there were no detectable differences in their exposure to predators, as determined by vegetative cover, in the absence of ungulates. These results demonstrate that ungulates can have strong and rapid impacts on small mammal abundance and diversity in East African savannas, an interaction which has not previously been given serious consideration.