Abstract In tropical countries, a large number of finfish species and invertebrates are captured as by-catch, including several species of crustaceans, due to the low selectivity of the fishing methods. By-catch presents and optimal opportunity to study populations of crustaceans. Here, the goal was to determine the community structure, specifically through the size structure, as well as the average sizes at first maturity of crustacean species present in the shrimp by-catch caught aboard a fishing vessel operating in the Mexican Pacific, from Sinaloa to Guerrero, Mexico. From the 18 crustaceans found in this study, the crabs Euphylax robustus and Achelous asper were the dominant species, whereas four species were considered casual (i.e., not frequent). Interestingly, more than 40% of the organisms analysed were in the juvenile stage, and 19% were ovigerous females. The results demonstrate the negative impact of the low selectivity of shrimp trawls on the communities of species of Crustacea.