The Fox River, Oshkosh, Wisconsin, has a history of fish kills, dating back to the late 1950s, with at least 58 fish kills occurring during April–October 1988. Twenty fish species, including lake sturgeonAcipenser fulvescens, were affected. In situ bioassay tests of fathead minnowPimephales promelasidentified the origin of toxicity to be a motor endurance testing facility. Additional testing showed carbon monoxide (CO) to be the cause of toxicity. Blood samples taken both during in situ bioassays and from captured fish during fish kill episodes showed blood carboxyhemoglobin (COHb) levels of 32–97%. Fish kill episodes were enhanced by flow reversals in the Fox River caused by seiches from 55,730-ha Lake Winnebago. The motor endurance testing facility remains in operation; however, testing has been reduced and is monitored for compliance.