Water-soluble root extracts of the Mexican marigold, Tagetes minuta L., composed mainly of the compounds alpha-gurjunene, 5-(But-3-ene-1-inyl)-2,2′-bithiophene, palmitic acid, alpha-terthienyl, and 5-(4-Acetoxy-1-butenyl)-2,2′-bithiophene, were assessed to determine their impact on six species of nontarget aquatic invertebrates. Test organisms were collected from a polluted stream—Gammarus lacustris Sars (Amphipoda)—as well as from a local nearly pristine stream—Lepidostoma pluviale (Milne) (Trichoptera), Drunella grandis (McDunnough) (Ephemeroptera), Baetis tricaudatis (Dodds) (Ephemeroptera), Rhithrogena morrisoni (Banks) (Ephemeroptera) and Hydropsyche cockerelli (Banks) (Trichoptera). Root extract of T. minuta, formulated with a commercial combination of petroleum solvents and surfactants, caused mortality to some of the organisms tested. Some mortality was attributable to presence of the inert, surfactant/petroleum-based solvent. For example, with L. pluviale, the LT₅₀ for 4 ppm marigold root extract formulated with 0.01 ppm inert materials was 54.7 h (CI 48.1, 64.2), and for 0.01 ppm inert materials alone was 69.8 h (CI 55.8, 101.2). Of the macroinvertebrates tested, R. morrisoni was the most sensitive to inert materials (LT₅₀ 0.01 ppm inert = 4.5 h) and G. lacustris was the least sensitive (LT₅₀ 0.01 ppm inert = 400 h; LC₅₀ 96 h = 1.2 ppm). Marigold root extracts did have a toxic effect at the highest concentration (4 ppm) used on all macroinvertebrates tested. Although previous studies reported the effects of one compound in the root extract, alpha-terthienyl, our research is the first published report evaluating effects of full-formulation marigold root extract on immature aquatic nontarget insect species and a crustacean. If root extracts of T. minuta are to be used as a pesticide, further research should focus on effects of inert surfactants used in marigold extract formulation on nontarget macroinvertebrates.