A new bioassay using larvae of the pitcher-plant mosquito, Wyeomyia smithii, was used to assess water for the presence of toxic contaminants. Procedurally, the bioassay has the advantages of simplicity, minimum equipment requirements, and low maintenance. It is well suited for bioassays of water from remote sites because of the minimal amount of water (30 mL) necessary for each replicate. The bioassay was capable of detecting 30 ppm of 1 Stroke Ves-Phene (a disinfectant detergent concentrate containing phenates), 5 ppb of heptachlor epoxide, and 100 ppb of malathion. In addition to reducing survival, toxicants caused delayed development and produced signs of intoxication unique to each substance. In another instance, the bioassay was used in the investigation of a fish kill. Persistence of toxicity, delayed development, and non-nervous signs of intoxication indicated which samples were contaminated and suggested that heavy metals were involved. Subsequent analysis revealed toxic levels of copper and elevated levels of zinc and manganese.