The insect growth regulators, diflubenzuron (=Dimilin tm) and Methoprene (=Altosid 10F) applied at a target dosage of 1 ppm to larval habitats of Culex quinquefasciatus in a crowded section of Jakarta (trial area 1 km) were highly effective in preventing successful adult emergence for 2 and 5 weeks respectively after one application. Ten days after spraying with Dimilin, tarsal abnormalities were noted in emerged adults which coincided with a preponderance of emerged males.