This study compares the demographic characteristics of two tropical heteromyid rodents: Liomys salvini, which inhabits deciduous (seasonal) tropical forests, and Heteromys desmarestianus, which inhabits wet (relatively nonseasonal) tropical forests. The hypothesis is examined that, because of the stronger seasonality of its environment, L. salvini is more r—selected than H. desmarestianus. Mark—and—release techniques were used to document population events occurring between August 1970 and August 1971 and again in June and July 1972 at two localities in lowland Costa Rica. Density fluctuated annually in both species, with the mean density of H. desmarestianus being much higher than that of L. salvini, probably because of the greater productivity of its habitat. Reproduction is more strongly seasonal in L. salvini, and females mature faster and produce larger litters than those of H. desmarestianus. Survivorship appears to be higher in the latter species. Adults of L. salivini increase markedly in weight during the breeding season. Individuals of both species occupied home ranges that were relatively stable in size aNd location for extended periods (1 yr or more). There were seasonal differences in the distances moved between successive capture sites in males of both species. Owing to earlier maturity and greater productivity per month of breeding, L. salvini appears to be more r—selected than H. desmarestianus. Many of the demographic characteristics of these tropical forest—dwelling heteromyids are similar to those of their temperate zone desert—dwelling relatives.