Digenea are common platyhelminth parasites in intertidal organisms; their free-living stages (mirac idia and cercaria) are not free of experiencing the effects of physico-chemical variations of this environment, same as their hosts. Moreover, the hosts may show different physiological behavior depending on if they are or not parasited. This study deals with the effects that a yet unidentified digenean may have upon the oxygen consumption of Perumytilus purpuratus, as well as the effects that may have the emersion-immersion cycles in the laboratory, on cercariae release. To do this, individuals of P. purpuratus were collected by hand from intertidal rocky platforms nearby the mouth of the Biobio River, Chile. In the laboratory, oxygen consumption was compared between hosts with and without parasites, and the sheddi ng of cercariae was measured at 9, 17 y 22 degrees C and compared between parasitized hosts exposed to opposite emersion-submersion cycles. There were no significant differences between parasitized and non-parasitized hosts in oxygen consumption. However, parasitized hosts showed scarce gonadic development, suggesting that metabolic products of the hosts are re-allocated towards parasite development. Cercariae shedding increased with water temperature and decreased exponentially with the duration of the submersion-emmersion cycle, peaking 2 h after submersion. These results suggest that metabolic rate of the host increases with temperature and that cercariae release is induced by temperature and submersion of the host in seawater.