The association between the clam Mulinia edulis and the isopod Edotea magellanica was studied during the Spring-Summer period of 1987-1988 at the subtidal bottoms of sand bars located outside the outlets of Lingue and Queule estuaries (southern Chile), and at the subtidal muddy-sand sediments of the middle reaches of these estuaries. The percentage of bivalve occupation by isopods at those estuaries varied between 1.5-4.2%, while at the sand bars the ranges were 21.1-80.7%. Laboratory experiments suggested that the low frequency of isopod occupation in estuarine clams could be related to the low tolerance of Edotea magellanica to water salinities lower than 20 parts per thousand. The highest percentage of isopod occupation in the clams collected from the sand bars occured in bivalves with shell length ranging from 50 to 60 mm. Regression and covariance analyses for the relationship between shell length and dry-tissue weight of clams with and without isopods, showed that Edotea magellanica has no influence on the meat content of clams. Isopod females outnumbered males at both sand bars. Number of intramarsupial eggs and embryos was significantly correlated with sizes of ovigerous females, while no significant correlation was found between number of intramarsupial juveniles and sizes of ovigerous females. In general, no correlation was found between size of intramarsupial stages and sizes of ovigerous females. The mean size for eggs in the whole sample was 0.55 mm, 0.96 mm for embryos and 1.51 mm for intramarsupial juveniles. The fife cycle of Edotea magellanica was schematically constructed using the available data. Clams would be first occupied by a female, then by a male. The new isopod generation develops in the marsupium of females and continues in the mantle cavity of clams until they reach an approximate body size of about 4 mm. The low number of juveniles found in the mantle cavity of clams suggests that these stages have a high mortality after released from the marsupium, or that they do not leave the clams at the same time. No isopods were ever seen outside the mantle cavity of clams.