Field surveys showed that mosquito larvae are rarely found in otherwise suitable breeding places when the water surface is densly covered with the duck weed, Lemna species. Laboratory tests using colonized Culex Pipiens indicated that scattered mates of the plant attracted more females to oviposit and enhanced larval development but obstructed egg hatching. Dense plant cover seemed to prevent Larval respiration and adult emergence. The results also indicated the absence of toxins or metabolic by-products harmful to Larvae in the breeding water of Lemna plant.