Amphidinolides A 111, B 121, C 141, and D 151, novel macrolides with potent antineoplastic activity, have been isolated from the cultured cells of the marine dinoflagellate Amphidtnium sp., a symbiotic microalga in an Okinawan flatworm. The structures have been assigned mainly on the basis of 2D nmr. Some compounds with interesting pharmacological activities have been also obtained from the cultured cells of other marine microalgae, symbionts of coral or bivalve. Cultivation of microalgae and isolation and structure elucidation of the bioactive compounds are described.