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This study aimed to investigate the population biology of Minuca mordax in the Igaraçu River, Parnaíba, analyzing frequency distribution in size classes, reproductive period, relative growth, morphological sexual maturity, sex ratio and heterochely. The crabs were manually collected, randomly, by two people for 1 hour, in the representative months of the dry season (September to November) and rainy (April to June). In laboratory the crabs were identified, separated according to sex and measured. A total of 610 samples were captured, with 453 males and 157 females. The average size of carapace width (CW) was significantly different (p < 0.05) between males (17.21 ± 3.68 mm) and females (15.06 ± 3.18 mm). The distribution of the crabs in size classes was unimodal for both sexes. The morphological sexual maturity obtained for males was 12.42 mm CW and females was 10.05 mm CW. The sex ratio for the population was 1:0.35 (male: female), with significant difference from the expected ratio of 1:1. The heterochely was observed only in males of the species (224 right-handed and 229 sinister). Subsequently, other on-site observations were carried out monthly, from February/2011 to January/2012 to observe ovigerous females and determine the reproductive period. Females with eggs were observed only from March to June, corresponding to the rainy season in the region. The results found in the present study are similar to those observed in other populations of violinist crabs and suggest that the population is stable in the environment. The present study provides subsidies for the conservation of the species in the Igaraçu River.