The Microtox test was evaluated as a tool to measure the presence of phorate, an organophosphate insecticide, in northern prairie wetlands. Microtox test results were inconsistent with results of otherin situtoxicity tests with aquatic invertebrates and ducklings. The Microtox test often indicated no toxicity, even though both invertebrates and ducklings died. Caution should be used when interpreting the results of the Microtox test to evaluate wetlands for pesticide contamination.