This study analyzed a comparison of the Atlantorchestoidea brasiliensis (Crustacea: Amphipoda) density of a preserved area with that of an urbanized one, both on Peró Beach, Cabo Frio, Rio de Janeiro state. Seasonal samplings were conducted of the population of talitrideos and an estimation of the number of swimmers in these areas was made. The lowest frequency of swimmers was recorded in the preserved area regardless of sampling period. In the urbanized area swimmers were most frequent in

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... ry (550 individuals/m-1). The statistical tests showed the absence of Atlantorchestoidea brasiliensis in the urban area, indicating that this species is extremely sensitive to the urbanization of beaches. The use of the Talitridae family as an indicator of the impact of urbanization on the ecosystems of sandy beaches is also discussed.