Brown mussels Perna perna (Linnaeus, 1758) are of great economic and social importance in the aquaculture and fishing sectors in many Brazilian states. The present study aimed to compare the sizes at first sexual maturity of brown mussels (L₅₀) from natural banks exposed to different levels of anthropogenic pressure. Mussel spat were extracted along the coast of the state of Espírito Santo, Brazil, and were secured on longlines at ilha dos franceses. The human collection pressure at each location was assessed through identification of shellfish gatherers at these sites. Over a 3-mo period, 70 individual mussels from each location were removed each month for macroscopic and microscopic analysis of gonadal development. The sex ratios of the populations at Vila Velha, Anchieta and Piúma showed alternating values between the sexes. Histological analysis confirmed all of the macroscopic observations regarding sex determination and the classification of sexual stages. The average sizes at first sexual maturity of male brown mussels from different spat collection sites ranged from 22.6 to 25.2 mm. The collection pressure exerted on natural banks of Perna mussels differed significantly between the sites studied. On the natural banks where the differences in human collection pressure were recorded (at Vila Velha and Piúma), the size at first sexual maturity significantly differed for both males and females. Therefore, the results suggest that the differences in size at first maturity were due to anthropogenic pressure.