a survey of mytellaguyanensis occurrence and population dynamics were performed in the paraíba do norte river estuary. the characterization and the influence of temperature, salinity and substrate, as well as the associated fauna and flora on the population were also examined. mytella guyanensis lives buried in the substrate of the mangrove intertidal zone of the paraíba do norte river estuary, from the mouth to 24 km upriver. average annual density of this bivalve species was 5.2 individuals per m2, with a predominantly aggregated distribution. the most frequent size was between 4.5 and 5.5 cm in length. analysis of the substrate demonstrated the presence of two types of substrates in relation to the percentage of silt and clay and the water content. besides interfering in the population density, the substrate heterogeneity may be a decisive factor in aggregation. eight species of crustacean decapods and one bivalve species were identified as associated fauna.