We investigated the effects of food availability, fire, and habitat structure on population rates of increase of the rodent Bolomys lasiurus in forty 4-ha plots distributed over an area of about 200 km² in Amazonian savannas near Alter do Chão, Pará, Brazil. Mean rodent density over the entire study area declined by about one-half during the study period, despite largely independent density fluctuations within plots. Fires had no detectable association with density and population growth rates. Both densities and population growth rates were significantly associated with availability of invertebrate prey and a multivariate index of habitat structure. Invertebrate availability varied temporally within plots, and was not predictable from knowledge of vegetation structure or fire events.