The mosquito Culex pipiens L. is an important pest in urban and suburban areas in many parts of the world. Septic tanks are the most important habitats supporting the production of this species in the city of Antalya, southwestern Turkey. Diflubenzuron, in a 25% wettable powder (Du-dim (R) 25 WP), and a 4% granular formulation (Du-dim (R) 4 G) was evaluated against late 2nd to early 3rd instars of Cx. pipiens in single-family septic tanks. Both formulations were tested at 0.01, 0.02, and 0.03 mg (AI)/liter. The results indicated that both formulations applied at the rate of 0.02 and 0.03 mg (AI)/liter achieved 100% adult inhibition, at intervals of 7, 14, 21, and 28 days after treatment. Septic tanks treated with 0.01 mg (AI)/liter WP formulation resulted in complete (100%) adult inhibition through 14 days, whereas the G formulation gave the same effect through 21 days posttreatment at this rate.