We investigated the generation of active oxygen compounds in the bivalves, Crassostrea gigas, Fulvia mutica and Tridacna crocea in order to understand the defensive mechanisms in giant clams that allow a stable association with symbiotic zooxanthellae. Production of superoxide anion was determined by MCLA-dependent chemiluminescence. Produced nitric oxide was determined by assay of fluorescence of 2,3-diaminonaphthotriazole formed from 2,3-diaminonaphthalene and nitrite (a stable end product of nitric oxide). C. gigas produced active oxygens, superoxide anion and nitric oxide upon stimulation by phorbol myristate acetate. F. mutica generated a small amount of superoxide anion and nitric oxide, and contained significant phenol oxidase activity which catalyzed the formation of quinones. T. crocea did not generate any apparent active oxygen compounds or quinones. In bivalves, the symbiosis is observed only in some clams belonging to Veneroida which lack or have weakened production of active oxygens. This suggests that low oxygen cytotoxicity may be a prerequisite for the microalgae to establish symbiosis with the animals.