The side effects of candidate mosquito control agents against nontarget organisms were investigated in the laboratory and outdoors in artificial containers, intermittent ponds, and irrigated pastures. The compounds used were TH 6040 [1-(4-chlorophenyl)- 3-(2, 6-difluorobenzoyl)-urea], H 24108 [3-butyn-2 ylN- (p-chlorophenyl) carbamate], and Altosid® [isopropyl 11-methoxy-3, 7, 11-trimethyl-2, 4 dodecadienoate].In the laboratory, crustaceans, especially the tadpole shrimp,Triops longicaudatusLeConte, clam shrimp,Eulimnadiaspp., and water fleas,DaphniaandMoinaspp., were susceptible to TH 6040 at levels below 0.01 ppm. Copepods,CyclopsandDiaptomusspp., showed some tolerance at 0.01 ppm, whereas seed shrimp,CypricerusandCypridopsisspp., were tolerant to as much as 0.5 ppm. Among aquatic insects tested, mayfly nymphs,Callibaetisspp., and chironomid midge larvae,Goeldichironomus holoprasinus(Goeldi), were sensitive. However, beetles,Thermonectus basillaris(Harris),Laccophilusspp.,Hydrophilus triangularisSay, andTropisternus lateralis(F.), demonstrated a strong tolerance. Mosquitofish,Gambusia affinis(Baird and Girard), also showed no effect at high dose levels.In artificial containers, TH 6040 suppressed the cladocera and copepod populations, but they recovered again. There was no effect on seed shrimp and the sideswimmer,Hyalella azteca(Saussure). TH 6040 also suppressed reproduction of the backswimmer,Notonecta unifasciataGuerin. There was no adverse effect on the backswimmer and mosquitofish colonies when the SR-1O formulation of Altosid was applied.In field tests, TH 6040 suppressed populations of tadpole shrimp, clam shrimp, water fleas, and copepods; but these recovered soon. Adult emergence of mayflies and aquatic midges was affected by TH 6040 treatments. Some affected midge pupae were noticed when Altosid was applied.