We I4C-labeled cellulolytic bacteria and 3 species of bacterivorous nanoflagellates and fed these cultured organisms to 2 species of intertidal mussels, Geukensia demissa and Mytllus edulis. using a pulse-chase experimental design under controlled laboratory conditions. Ingestion and assimilation of C from these rmcroheterotrophs by mussels were calculated from measured rates of defecation, respiration, excretion, and tlssue incorporation. The proportion of available C ingested by G. demissa did not dlffer significantly among bacteria (39%), heterotrophic flagellates (58%), or the unicellular algae Isochrysis galbana (66%), which was used as a reference diet. In contrast, M. edulis ingested a signlflcantly lower proportion (19%) of the small bacterla ( < l pm in diameter) than the larger ( 3 to 5 pm diameter) heterotrophic flagellates (58%). The efficiency with which G. dernissa assimilated C from I. galbana (77 %) was significantly greater than that from either bacteria (42 %) or heterotrophic flagellates (44%) M edulis assimilated bacterial C with significantly lower efficiency (21 96) than C from heterotrophic flagellates (62%) These results indicate that heterotrophic flagellates can contribute to the C requirements of both G. dernlssa and M edulis, however, only G. demissa is capable of assimilating a considerable amount of C from bactena.