Introduction: Macrobrachium amazonicum is a species of shrimp native to South America, widely distributed and exploited by artisanal fishermen in the North of Brazil. Objective: This study aimed to analyze the population structure of M. amazonicum, with an emphasis on sex ratio, frequency distribution of male and female size, also considering the rainy and dry period in the study areas. Methods: For this, monthly collections were carried out from May 2017 to April 2018 with the help of “matapis” in two locations, Mazagão and Ilha das Marrecas. Results: A total of 6 796 (4 163 females and 2 633 males) were captured, with a monthly average of 531.66 specimens. The sex ratio in both areas was favorable to females with 1.32♀ : 1♂ (x² = 76.58, p < 0.001) in Mazagão and 2.34♀ : 1♂ (x² = 394.26, p < 0.001) in the Ilha das Marrecas. In both Mazagão and Ilha das Marrecas, abundance was strongly related to precipitation. Conclusions: The shrimps of Ilha das Marrecas were longer than those caught in Mazagão. Despite this fishing pressure in Mazagão, the areas proved to be favorable for the growth of the species.