The study of the population structure of A. armillatus was based on monthly collections (March 2000–February 2002) from two tidal flats—São Francisco Beach (SF), São Sebastião County, and Engenho d'Água Beach (EA), Ilhabela County—located at the northern portion of the State of São Paulo coast and separated by the 6 km wide São Sebastião Channel. Carapace length (CL) was measured to the nearest 0.01 mm. The proportion of ovigerous females and the embryonic development stage of eggs were recorded. The Von Bertalanffy growth equation was fit to modes of the temporal sequence of frequency distributions of CL classes (SF data). The same sampling effort yielded 547 males and 566 females at SF and 145 males and 150 females at EA. Proportions of ovigerous females were higher than 60% throughout the year in both sites. The temporal trend of percentage of late eggs indicated three annual hatching periods. Growth patterns of males and females were similar, with the former tending to grow more rapidly (K = 2.28) and to attain smaller size (L_∞ = 13.90) than females (K = 1.90; L_∞ = 14.90). The estimated longevity of males and females was 1.20 years (14.4 months) and 1.29 years (15.5 months), respectively.