We evaluated the acute toxicities of the main degradation products of pyridine triphenylborane (PTPB), namely, diphenylborane hydroxide (DPB), phenylborane dihydroxide (MPB), phenol, and biphenyl, to the alga Skeletonema costatum, the crustacean Tigriopus japonicus, and two teleosts, the red sea bream Pagrus major and the mummichog Fundulus heteroclitus. DPB was the most toxic of the degradation products to all four organisms. The acute toxicity values of DPB for S. costatum, T. japonicus, red sea bream, and mummichog were 55, 70, 100, and 200-310 μg/L, respectively. The degradation products were less toxic than PTPB to S. costatum and T. japonicus; however, the toxicities of DPB and PTPB to the fish species were similar. We also examined changes in the inhibition of growth rate of S. costatum as well as the percentage of immobilization of T. japonicus as end points of toxicity of PTPB after irradiation of PTPB with 432 ± 45 W/m(2) of 290-700 nm wavelength light. After 7 days of irradiation with this light, the concentration of PTPB in the test solutions decreased markedly. A decrease in toxic effects closely coincided with the decrease in the concentration of PTPB caused by the irradiation. PTPB probably accounted for most of the toxicity in the irradiation test solutions. Because the concentrations of PTPB that were acutely toxic to S. costatum and T. japonicus were <10 % of the corresponding concentrations of its degradation products, PTPB probably accounted for most of the toxicity in the irradiation test solutions.