The objective of this work has to get knowledge about the reproductive aspect of the tree crab Aratus pisonii (H. Milne Edwards, 1837). The sampling were made monthly from April, 2000 to April, 2001, in the Itacurucá's mangrove (22º54'S and 43º33'W), Rio de Janeiro. The crabs were collected handly, using an effort of 50 minutes with three collectors. In laboratory, the specimens were separated by sex and had measured the carapace width and lenght. The ovigerous females were separated and the mass of eggs was removed for a mensurament of size and count of eggs. Afterwards, the animals were conservad in 70% alcohol. A total of 783 individuals were caught among 59% males and 41% females among these 8% were ovigerous. The ovigerous females were found almost during the whole year, except in the winter showing major plenty on February and March, demonstrating the reproductive period. The ovigerous females presented size between 14.0 and 23.6 mm of carapace width and the average fecundity estimated to the specie was 9081 ± 3684 eggs. The mean size of the eggs was 0.27 mm varied from 0.24 to 0.29 mm and the size at onset of egg production was verified from 13.1 to 15.0 mm of the carapace width.