AbstractBacterial degradation of the cyanobacterial cyclic peptide hepatotoxin microcystin was confirmed in natural waters and by isolated laboratory strains. Degradation of 1 mg L−1 microcystin LR typically began 2‐8 days after addition to surface water samples. At concentrations greater than 1 mg L1 there was an initial slow removal of microcystin LR, rather than a distinct lag (or conditioning) phase, before rapid degradation commenced. The lag phase was absent upon re‐addition of microcystin LR to the water. Both single strains and mixed bacterial cultures capable of degrading microcystin LR were isolated from surface water samples. One single strain isolated was a gram‐negative rod and appeared to be a Pseudomonas sp., although standard taxonomic tests have given inconclusive results. Degradative activity was mostly intracellular and equally active against microcystin LR and RR, but not against nodularin. © 1994 Wiley‐Liss, Inc.