Density and biomass may reveal different aspects of the dynamics of populations, but most studies have focused on density or relative abundance. Density and biomass also may behave differently in parts of the population composed of males and females because of differences in vagility and parental care between sexes. Herein, we explore seasonal and multiannual variation in density and biomass in a population of black-eared opossums (Didelphis aurita). Females and males were analyzed separately for associations with precipitation and litterfall, indicators of resource availability. Litterfall, density, and biomass were estimated from 1997 to 2003 on three 0.64-ha trapping grids in an area of Atlantic Forest in Rio de Janeiro, Brazil. Density of females exhibited a significant increase during the study. Density and biomass of males were more variable, without any significant positive trend or correlation with precipitation or litterfall. Seasonal and biannual patterns of precipitation were detected, followed by the density and biomass of females with time lags varying from 0 to 1, and from 9 to 12 months. The relative stability of biomass of females, and its association with precipitation and litterfall, suggests that resource availability combined with density-dependent responses regulated the local population of females. The local population of males may be more influenced by the stochasticity resulting from the higher vagility of males.