Mosquitoes are specific and selective about their breeding places, and the physico-chemical factors of the breeding of the breeding pools play a vital part in the selection of the oviposition site and subsequent growth development and population density during the aquatic stages. The present study of the physico-chemical factors conducive to selection of breeding places of Culex quinquefasciatus=fatigans reveals that the positive factors for the high population density of the aquatic stages of this species are the high concentration of total soluble salts, free ammonia, organic carbon nitrates and slightly alkaline pH. The negative factors are their low concentration and pH above 8. In the thin line separating the positive and negative factors present in the physico-chemical composition of the water of the breeding places may lie the key to the practical control of Culex quinquefasciatus=fatigans which has to be determined exactly by vigorous efforts in that direction.