The population structures of Aratus pisonii and Sesarma rectum were analysed in order to compare their life histories. These two sinchronotopic species occupy distinct mangrove microhabitats in Ubatuba, São Paulo, Brazil. In order to compare the population dynamics of these two species, age and reproductive status of crabs were estimated by obtaining monthly collections over a 2-year period. Aratus pisonii follows an annual population cycle. Highest frequencies of ovigerous females were recorded during spring and summer, while juvenile recruitment peaked during the winter when the adult population was less representative. The life span of A. pisonii was estimated to be one or two years. On the other hand, recruitment of S. rectum was continuous throughout the year. Adults were consistently present in all months, suggesting that the life span of S. rectum is longer than that of A. pisonii, and its reproduction might be continuous. Temperature is correlated with frequency of breeding females and juveniles in A. pisonii, while no such correlations are found in S. rectum. These results suggest that the population dynamics of these two grapsid crabs is considerably different.