A field studies were conducted in Burlington County, New Jersey during 1975 and 1976 to determine the applicability of Gambusia affinis to urban mosquito control. Two urban stream systems with varying degrees of pollution were stocked in an attempt to control larvae of Culex pipiens. Periodic water analyses, stomach dissections, and larval surveys were performed. A June stocking of 0.65 fish/meter in one stream resulted in 100% control of Cx. pipiens in 4 weeks. In both streams Gambusia were observed 3 miles downstream from stocking sites within 8 weeks. They inhabited and successfully controlled larvae in known breeding areas.