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The structural complexity of algae is an important factor driving the structure of epifaunal assemblages associated with marine macroalgae. We tested how the structural complexity of four Sargassum species is related to the structure of gammarid assemblages. We measured different algae traits (frond length, number of branches, and epiphytic hydroid and algae cover) and different ecological descriptors of gammarid assemblages (richness, density, diversity, and evenness). Samples were obtained in southeastern Brazil in early and late summer. Structural complexity was associated with the occurrence of significantly different gammarid assemblages. Sargassum cymosum had more branches and greater hydroid coverage, and a higher gammarid species richness, diversity, and density. The characteristics of the gammarids inhabiting the different Sargassum species were similar between sampling periods, indicating that the relationships between the Sargassum traits and associated gammarids are strong and persistent. Species richness, diversity, and density were all more dependent on habitat heterogeneity (number of branches and coverage of epiphytic hydroid) than the quantity of habitable space, i.e., frond length. Overall, the variation in structural complexity among Sargassum species had a significant effect on gammarid assemblage structure. [ABSTRACT FROM AUTHOR]