Pyriproxyfen is a new insect growth regulator with potent activity against developing stages of the cat flea, Ctenocephalides felis. To determine its duration of action after application as a spot-on topical treatment, 20 cats were allocated to two groups. One group acted as untreated controls, and each cat in the other was given a single dose of 1 mg/kg pyriproxyfen. Each cat was infected with fleas at weekly intervals and caged. Eggs were collected and their viability determined as a measure of the effect of the agent on adult fleas. The effect of the transfer of pyriproxyfen from each cat to its immediate surroundings was monitored by observing the development of fertile flea eggs placed on samples of blanket taken from each cage. Results from untreated control cats showed that the experimental conditions were favourable for the lifecycle of the flea. Flea reproduction on treated cats was inhibited completely for seven weeks and substantially for at least a further two weeks. Blankets used by cats treated up to two weeks previously prevented flea development entirely and substantial control was still achieved up to four weeks after treatment.