Bianca Sartini, Roberto Marchesini, Sthefane D ´ávila, Marta D'Agosto, and Roberto Júnio Pedroso Dias (2018) Freshwater gastropods represent good models for the investigation of epibiotic relationships because their shells act as hard substrates, offering a range of microhabitats that peritrich ciliates can occupy. In the present study we analyzed the community composition and structure of peritrich epibionts on the basibiont freshwater gastropod Physa acuta. We also investigated the spatial distribution of these ciliates on the shells of the basibionts, assuming the premise that the shell is a topologically complex substrate. Among the 140 analyzed snails, 60.7% were colonized by peritrichs. We observed seven peritrich species: Epistylis plicatilis and Epistylis sp. (Epistylididae); Opercularia articulata (Operculariidae); Carchesium polypinum, Vorticella campanula and Vorticella sp. (Vorticellidae) and Thuricola kellicottiana (Vaginicollidae). We observed a high prevalence of epibiosis (> 60%) when all species of ciliates were considered conjunctly. However, the prevalence was low (1-58%) when each species was considered separately, reflecting their aggregate distribution pattern. The most prevalent species were Epistylis sp. (58.60%), Vorticella sp. (14.30%) and O. articulata (13.60%). Although the epibionts were distributed through the shell's entire extension, we observed greater values of abundance, density, diversity and dominance on the dorsal surface. Only Epistylis sp. was widely distributed on the ventral surface. We also observed that the peritrichs predominantly occupied the areas of the shell delimited by the sutures. We interpreted these distribution patterns considering that the peritrichs select their attachment sites under pressures related to basibiont behavior, physical forces that may dislocate them and protective characteristics of the shell's areas.