During a 14 month mark‐and‐release project in an Atlantic forest area (Juréia Ecological Station), I studied the population ecology of 8 species of small mammals (3 marsupials and 5 rodents). In a 4,307 trap nights, 182 individuals were captured 650 times. The rodents were caught 73% of the total capture and the marsupials 27%. Oryzomys nitidus was the commonest species in the grid followed by Proechimys iheringi and Metachirus nudicaudatus. The marsupials showed a seasonal pattern of reproduction. As a consequence, the population densities had a peak during the wet months with the entrance of the youngs. The rodents, P. iheringi and O. nitidus bred throughout the year, although the former one had a peak in the dry months. Density of O. nitidus was fairly constant, whereas that of P. iheringi was higher during the recruitment time. Rodents and marsupials contributed with the same biomass in Juréia small mammal community.