A bacterium termed 7CY, capable of decomposing cyanobacterial toxins, was isolated from surface water sample of Lake Suwa and degradation of microcystin-RR and nodularin-Har was investigated. The isolated 7CY was a gram-negative, aerobic bacillus, and a member of a genus Sphingomonas. The strain degraded microcystin-LY, -LW, and -LF completely as well as microcystin-LR within 4 days after their addition (6 microg/ml) whereas degradation of nodularin-Har did not occur at all during experiment. On the contrary, the strain was capable of degrading nodularin-Har in the presence of microcystin-RR and both toxins were completely decomposed within 6 days. The strain scarcely degraded nodularin-Har in the presence of microcystin-RR when glucose and ammonium chloride were added to the medium. The degradation of nodularin-Har did not occur in the medium from which bacterial cells had been removed after degradation of microcystin-RR. Furthermore, when microcystin-RR and nodularin-Har were added to the cytoplasm fraction of 7CY cells, microcystin-RR was rapidly degraded within 18 h, but nodularin-Har was not. The strain 7CY may require an enzyme(s) induced during the degradation of microcystin-RR in order to utilize nodularin-Har as nutrition.