After distillation, the autophagostimulant activity ofCulex pipiensL. larval water (millipore-filtered water in which kaolin-glutted larvae have been held overnight at a density of 1–3/ml) was found in the residue, whereas distillate fractions were inactive. After washing larval water with ether, virtually all activity was found in the aqueous phase. These separatory characteristics distinguish theC. pipiensautophagostimulant from growth-inhibiting factors produced by larvae of other mosquito species. More potent phagostimulant activity, also ether-insoluble, occurred in overcrowded larval water (10 larvael/ml, with restricted surface area and food provided for 3 days), but no evidence was obtained that such waters contained a growth-inhibitory factor of the sort produced byCulex quinquefasciatusSay larvae. Water in whichC. pipienspupae or eggs had been held overnight sometimes stimulated increased larval ingestion, but generally the activity was trivial vs. that of larval water. Larval, pupal, and egg waters elicited preferential oviposition in choice tests vs. distilled water. For larval water, oviposition-inducing activity was most evident, after fractionation procedures, in the 1st distillate collected, showing that the larval autophagostimulant (not present in distillates) and the principal oviposition attractant are different materials. The ecological implications of the several mosquito-larval factors now known are discussed.