Specifically in Piedade reef, where we performed the present study, Isognomon bicolor invasive bivalve showed greater abundance in the mid intertidal band before dominated by the bivalve Brachidontes solisianus and the barnacle Tetraclita stalactifera. However, the abundance and size structure of Isognomon bicolor and Brachidontes solisianus are variable in the local collection related to desiccation and the presence of Ulva spp.. In this context, this study aims to evaluate the population structure of Isognomon bicolor and Brachidontes solisianus from density and shell length data. For that, were randomly collected 3 quadrats of 20 X 20 cm in February and May 2012, dry and rainy seasons respectively. The animals were scraped, and placed in labeled plastic bags, and frozen for later sorting. After sorting the length of the bivalve shells were measured with the aid of caliper, the seaweed Ulva spp. was also screened, dried and weighed. The Chi-Square test was used to assess the difference between the frequency distribution of the different size classes. The bivalves were divided into 10 size classes. The chi-square test detected a significant difference between the population structure of bivalves. Isognomon bicolor had lower density in May 2012 during the drought, while more individuals Brachidontes solisianus were found in May. However the largest proportion of individuals was always between 13-16 mm in both months of collection for Isognomon bicolor and 3 mm for Brachidontes solisianus presenting both bivalve unimodal distribution these sizes could be more resistant to desiccation and predation stress. The Mann-Whitney test showed a lower density for Ulva spp. in May 2012.