The phytal environment is a complex system that involves the association between marine organisms and macroalgae. In this paper, we investigate the diversity of bivalves associated with macroalgae on the continental shelf between the states of Alagoas and Bahia, including Sergipe, in northeastern Brazil. Macroalgae and associated fauna were collected during two sampling campaigns under the MARSEAL project (February and July 2011 [dry and rainy seasons, respectively]), covering 24 stations and three isobaths (10, 25 and 50 m). The following ecological descriptors were calculated: abundance (N), richness (S), diversity (H') and evenness (J). A total of 1384 individuals from 20 families, 28 genera and 44 species were obtained. Arcidae was the most abundant group, followed by the families Pteriidae and Mytilidae. The most abundant species were Arca zebra, Anadara sp. 1 and Pinctada imbricata, representing 71% of the total abundance. The families Arcidae, Corbulidae and Mytilidae were considered constant, as they occurred in more than 50% of the samples. A higher abundance was recorded during the rainy season. No seasonal differences were found regarding S, H' or J. Richness increased with increasing depth, whereas the other indices (N, H' and J) were not influenced by bathymetry. This reveals that the 50 m isobath has a greater support capacity and houses richer, more diverse fauna. Bivalve richness and composition data from this study expand the information on mollusk biodiversity associated with the phytal environment on the continental shelf off northeastern Brazil Ver menos