Large-scale outbreaks of two species of rodents (Oligoryzomys longicaudatus and Abrothrix olivaceus) in response to a large increase in food resources in San Martin Experimental Preserve and in the Peulla area of the Vicente Perez Resales National Park, were observed during fruiting and seedling of arborescent bamboo (Chusquea. spp.). This unique phenomenon enabled the analysis of the effect of increased density on patterns of habitat use by these rodent species with mark-recapture grids in each area. Vegetation analysis was performed at three strata at each capture site. Associations between habitat variables and rodents presence or absence was assessed by using a logistic regressions. Habitat preference was strongly influenced by population density. Vegetation variables were important predictors for the presence of these rodent species only in years of low numbers. The results corroborate the predictions of the Fretwell-Lucas model of habitat selection.