To study the efficacy of chemical larvicides; several materials were evaluated in two warm-water southern California artificial lakes, (Lake Calabasas and Westlake) and the residues of an effective larvicide (chlorpyrifos) in various components of one of the lakes were investigated.Abate®(O,O'-(thiodi-p-phenylene)O,O,O',O'-tetramethyl phosphorothioate) gave excellent control of Chironomine midges (ChironomusandDicrotendipesspp.) but only mediocre or poor control of the smaller Tanypodine midges (CricotopusandTanypusspp.). Carbaryl yielded poor control of both groups, and fenitrothion yielded good control of both groups at a higher rate of application.Chlorpyrifos gave excellent control of both groups of midges. It was more effective against the Chironomine larvae. This material at 0.2 lb active ingredients per acresurface yielded complete control of Chironomine midges for 12 weeks or longer, but suppressed the Tanypodine larvae for only 7–8 weeks.Residues of chlorpyrifos (used as 2% granules) were generally higher in the bottom water, even though maximum residues in both layers reached 0.6 ppb. Residues were much higher in the mud, reaching a mean maximum of 0.3 parts per million in the top 1-inch section of mud samples, but only traces were found in the remaining section of the mud. Maximum residues in water were obtained one day after treatment, declining to below 0.1 ppb within 4 weeks after treatment. The maximum residues in the mud were obtained one week after treatment, declining to below 0.1 ppm in 28 days. Slightly higher levels of residues were found in water and mud when the whole lake was treated.Bottom-dwelling and -frequenting fish, such as channel catfish (Ictalurus punctatusRafinesque) and, to some extent, black crappie (Pomoxis nigromaculatusLeSueur), contained much higher quantities of chlorpyrifos than the nonbottom dwelling fish such as largemouth bass (Micropterus salmoidesLacépède) and bluegill (Lepomis microchirusRafinesque). Mean maximum residues in channel catfish and black crappie were 0.8 and 0.5 ppm after 3 and 2 weeks posttreatment, respectively, declining to barely detectable levels 25 days after treatment. The residues in largemouth bass and bluegill were minimal, with maximum approaching 0.2 ppm but remaining 0.1 or below this level most of the time.