Four major toxic and growth-retarding compounds isolated from an overcrowded culture of larvae of the mosquitoCulex pipiens quinquefasciatusSay were chemically identified asn-heptadecane, 7-methyloctadecane,n-octadecane and 8-methylnonadecane. The minor constituents, presumed to consist of 2-ethyl long-chain fatty acids, also contributed to the overall activity of overcrowding factors produced by mosquito larvae.Production of these compounds was dependent on the larval density rather than on the number of larvae used. The “ego space” of the 4th-instar larva is ca. 9 mm2, below which the larvae come in bodily contact through the lateral thoracic and abdominal hairs, resulting in the elaboration of detectable quantities of biologically active materials.