Resumo. Abstract Aim The aim of this study was to investigate the isotopic signature of P. brasiliana captured in three different points along the river flow toward the sea of the Prumirim River, northern coast of the state of São Paulo/Brazil in order to test the hypothesis of similarity between isotopic signature of individuals living in separated areas of the river. Methods We used stable isotope analyses (δ13C and δ15N) at three points of the river and ANOVA and Bayesian Ellipses analyses were performed. Results Our results showed that are a differential isotopic enrichment along the river course in 13C, providing important results on the environmental condition and anthropogenic impacts in the region. In addition, the food biology of P. brasiliana was characterized as a primary consumer corroborating with the detritivores feeding habit observed in previous studies of stomach content analysis. Conclusions Our research, limited to a single area along the northern coast of the state of São Paulo, uncovers intriguing findings that merit replication in other areas within the region. This is particularly crucial given the rising number of anthropogenic influences resulting from urban development, underscoring the need for improved monitoring of these areas.