Brachycephalus didactylus, a leaf litter frog endemic to the Brazilian Atlantic rain forest, is considered the world's smallest tetrapod. Currently, there is little information about the ecology of this species. In this study, we estimated the population density and analyzed the diet, sexual dimorphism, and some reproductive aspects of two populations of B. didactylus in Rio de Janeiro State, Brazil. The estimated density was 4.0 and 1.2 ind/100m², suggesting that this is not a locally rare species. However, collecting these frogs may be difficult because of their very small body size (< 11.0 mm SVL) and cryptic color pattern. Individuals from the two populations did not differ in body size, but females from one population were significantly larger than were males. In both populations, the most important food items consumed were Acari and Collembola, most of them measuring less than 1.0 mm in length. Females of the two populations had 3.6 ± 1.3 and 2.5 ± 0.8 mature oocytes averaging 1.6 ± 0.5 mm and 1.4 ± 0.5 mm in diameter, respectively. Frogs with direct-development generally deposit a relatively small number of eggs, and this trend is even more extreme in miniaturized species, such as B. didactylus, which probably lays one egg per clutch. Because the body size of females significantly influenced the mean diameter of oocytes, but not the number of oocytes per female, reproductive success is probably associated with the production of relatively large eggs and, consequently, larger froglets, which may confer an adaptive advantage.