Residual effectiveness of the insect growth regulators pyriproxyfen and methoprene against cat flea, Ctenocephalides felis (Bouché), larvae were evaluated under simulated household conditions. Pyriproxyfen provided control of larvae for > 12 mo when applied at 1 mg and 0.2 mg/m2 and > or = 3 mo at the rate of 0.04 mg/m2. Methoprene applied at 1 mg/m2 provided control for > 12 mo and 6 mo at 0.2 mg/m2. No significant mortality was observed when methoprene was applied at 0.04 mg/m2.