The toxicity of an emulsifiable formulation of pirimiphos methyl (Actellic 25EC) on Anopheles gambiae s.s. Giles, Culex quinquefasciatus Say (Diptera: Culicidae), and predator fish Poecilia reticulata Peters (Pisces: Poeciliidae) was investigated. Acute toxicity tests were carried out to determine the effect of the larvicide on mosquito larvae and fish species. To investigate the nontarget effects on P. reticulata, fish of similar size (3.5 ± 0.2 cm) were randomly selected and exposed for 28 d, under static renewal bioassay, to sublethal concentrations of the larvicide capable of killing 30 and 70% of Cx. quinquefasciatus. The 24 h LC₅₀ value of pirimiphos methyl on the test organisms ranged between 20.44 and 697.30 µg liter^-1. The ultrastructural changes observed in the intestinal cells of P. reticulata were characterized by degenerating cell membranes with gradual loss of gray area in pycnotic nucleus at lower concentration. Marked damage was found at higher concentration including distinct loss of gray areas in cytosol, absence of cristae, numerous ruptures, and several dead cells. Pirimiphos methyl was toxic to a predatory fish species, and for its relevance in vector control and crop protection, warrants cumulative assessment to establish its comprehensive ecological risk, and the dosage required for field larviciding.