During the course of regular laboratory maintenance of colonies of Aedes aegypti and Culex quinquefasciatus, it was discovered that sugar cotton removed from cages of adults was lethal to all instars of larvae of both species as were aqueous extracts of the cotton. This same finding applied to field-collected material and unrelated to blood feedings. The activity of the toxic compound was of such magnitude that 2 female mosquitoes interacting with the sucrose cotton produced a larvicide which killed 100% of the specimens. The major peak obtained from high pressure liquid chromatography (HPLC) of the toxic extract was isolated, bioassayed, and its structure partially characterized from its ultraviolet, infrared and mass spectra. It is hypothesized that the adult female mosquitoes included in this study deposit a chemical while feeding on sucrose-soaked cotton, which interacts with it to form a potent larvicidal compound which is not species specific.