Malathion is an organophosphorus insecticide, which is often sprayed to control mosquitoes. When applied to aquatic habitats, malathion can also influence the embryogenesis of non-target organisms such as frogs and fish. We modified the frog embryo teratogen assay in "Xenopus" (FETAX), a standard toxicological assay, into an investigative undergraduate laboratory exercise. This exercise provided students with experience in developmental toxicology, experimental design, quantitative morphology, digital imaging, and presentation of research results. Their results demonstrated that "Xenopus" embryos exposed to malathion on the first or second day of development were indistinguishable from controls, while embryos exposed to malathion on the third day of development exhibited significantly bent tails and shorter body lengths. Similarly, sublethal malathion exposures also compromised early zebrafish development. To determine if this investigative laboratory exercise met its goal of fostering conceptual understanding of developmental toxicology, we compared student performance on a questionnaire before and after the laboratory exercise, which demonstrated significant improvement in conceptual understanding. Moreover, all (45/45) students successfully completed a modified FETAX and prepared posters of their results, indicating that students learned quantitative morphology and imaging skills while also gaining valuable experience in designing, executing, and communicating an experiment. (Contains 6 figures.)