ABSTRACTCertain blue-green algae commonly found in lakes can produce potent toxins during blooms. In Wisconsin there were several incidents over the years when wildlife and domestic deaths were attributed to algal toxins; however, little information existed on how widespread these toxins were in Wisconsin's many lakes and ponds. Thus, a study was undertaken to determine the incidence of measurable toxins in Wisconsin waters. Samples were collected from 86 lakes and ponds, the algae identified by genus, and the toxicity tested using a mouse bioassay. In 25 lakes toxins were present during bloom conditions. Lakes with toxins were found in both the southern and northern portions of the state. The results indicated toxic blooms were not uncommon in Wisconsin, and that algal toxins may be a more critical lake management issue than generally realized.