Population density and reproduction were studied in two leaf-eared mice (Phyllotis spp.) in the Peruvian locality known as Lomas de Lachay. Density varied between 0-3 individuals ha-1 in P. andium, and between 0-12 individuals ha-1 in P. amicus. An annual cycle of population abundance was observed in both species. Phyllotis amicus showed no seasonal reproductive activity, but P. andium had seasonal reproduction with pregnant females principally observed between July and September. Differences in juvenile weight were related to litter size. It is likely that two young was the optimal litter size in the harsh and seasonal xeric environment of Lomas de Lachay. Our results suggest that P. amicus maintains low, highly dispersed populations in a harsh environment, and can reproduce at any time of the year, whereas P. andium appears to be a more irruptive species closely tied to rainfall patterns.