Street catch basins in western Cook County, IL, were examined regularly June through August of 1987 to determine their seasonal larval production, adult mosquito harborage and the influence of abiotic factors on the associated mosquito population. Only larvae of Culex pipiens (63% of total) and Cx. restuans (37% of total) were recovered. These 2 species and Cx. erraticus were the most frequently collected adults. The abundance of mosquito larvae within catch basins was not correlated with water pH and only showed a weak, positive correlation with water temperature. Only a partial flushing of larvae (22-34% reduction) from catch basins by normal rainfall (less than 25 mm) was generally recorded. Treatments with larvicide oil (mineral seal oil/kerosene 175/tergitol) at a rate of 60-90 ml/catch basin resulted in a mean larval reduction of 97%. Adult females showed a mean reduction of 87%.