The insect growth inhibitor Alsystin (SIR 8514) was tested for its effects against the immature and adult stages of Culex quinquefasciatus Say under laboratory conditions. A linear correlation was revealed between concentration and larval and pupal mortality. Larval mortality increased from 5% at 0.005 ppm to 74.4% at 0.1 ppm of Alsystin. Complete inhibition of adult emergence resulted after larval exposure to 0.05 ppm. Exposure of eggs (at 0 hr of age) to 0.1 ppm and 10.0 ppm of Alsystin, resulted in considerable decrease of hatching from 87.0% in the control to 61.0% at 1.0 ppm and to 48.0% at 10.0 ppm. The adults of C. quinquefasciatus were seen attracted to the bowls containing 50.0 ppm and 100.0 ppm of Alsystin. However, landing adults died within a short period of time. The different types of deformation in the larva, pupa and adult as a result of exposure to Alsystin are illustrated.