Association Crepidula aculeata - Amphiroa beauvoisii. Population study of Crepidula aculeata Gmelin, 1791) (Gastropoda Calyptraeidae) and analysis of protocooperative association between the mollusc and the alga were carried out in the phytal of calcareous alga Amphiroa beauvoisii Lamouroux, 1816 at Ilha das Palmas, Santos Bay (24º00'S - 46º19'W). Samples were collected at three sites of different degree of water movement in the intertidal zone and sublittoral fringe during 1975 and 1976. From 93,916 organisms belonging to 172 species, 5,331 were Crepidula, the most abundant among sessile species. Also, Crepidula constitutes a substratum to attachment of Amphiroa, accounting for 30% of total phytal area. There was a positive correlation between the growth of shell and epizoic alga. Higher level of the phytal belt was more suitable for attachment and/or growth of the epizoic alga, without influence from water movement or season of the year. This association depends on the external morphology of the shell, filter-feeding habit, high adhesive power and lacking of mechanisms avoiding attachment of epizoics of Crepidula. Exuberant growth of the epizoic tuft over Crepidula female shell would constitute camouflage and protection of offsprings against predators.