The population dynamics and secondary production of the fiddler crab Uca rapax Smith, 1870 in the Itaipu Lagoon (Southeast Brazil) were studied. Samples were taken monthly over a period of 12 months, in order to test latitudinal gradient predictions about secondary production and P/B ratio. Males and females showed the same growth rates (K = 2.083) and reached the same asymptotic size (15.57 mm). However, females showed higher total (Z = 3.20) and annual mortality (A = 0.96 ind·year⁻¹) than males (Z = 1.89; A = 0.85 ind·year⁻¹). Recruitment occurred in all months, except in June, 2007. Secondary production was estimated in 113.4 g AFDW·m⁻²·year⁻¹ and the production to biomass ratio (P/B) was 15.09. The production and P/B ratio estimated in the present study did not agree with the latitudinal gradient literature predictions.