Seasonal variation of bivalve toxicity was monitored in association with the abundance of the toxic dinoflagellate Pyrodinium bahamense var. compressum in Masinloc Bay, Luzon Island. Among 7 species of bivalve, 6 species became toxic during a bloom of the dinoflagellate. However, remarkable difference in the toxicity was observed among the species. The toxicity of thorny oyster Spondylus squamosus was the highest among the species, showing more than 30 times that of safety consumption level after the peak bloom of the dinoflagellate, while other bivalve species showed much lower toxicity. The toxicity of thorny oyster decreased under absence of the dinoflagellate, but this species maintained a considerably high toxicity throughout a year. Similar trend was observed in penshell Atrina vexillum in a small scale, indicating that these species in the bay are not safe for human consumption almost throughout a year. The toxicity of green mussel Perna viridis increased to significant level during a bloom, but it decreased rapidly when the dinoflagellate disappeared. Toxin analysis of cultured and natural cells showed typical toxin profile of the dinoflagellate. Toxin profile of all the bivalve species reflected the characteristic toxin profile of the dinoflagellate.