Efficacy of polyvinyl chloride collars containing temephos [0,0'-(thiodi-4,1-phenylene) 0,0,0',0'-tetramethyl bis-(phosphorothioate)] was elevaluated in dogs and cats against the cat flea (Ctenocephalides felis). Over a period of 42 weeks, collared (treated) and control dogs were experimentally infested with 100 fleas 23 times, and control and treated cats were similarly infested 20 times during a 41-week period. Flea populations on treated dogs were reduced by 80% or more for at least 36 weeks. Initially, flea reduction by 80% occurred within 1 day, but in dogs wearing the same collars for 26 weeks, this reduction occurred in 3 to 6 or more days. Only during the first 2 weeks of wearing the collars were all fleas killed on all dogs within a 6-day period after infestation. Residual efficiency was lowest on highly active dogs and greatest on inactive dogs. The residual efficiency on cats was somewhat better than on dogs. At the end of 41 weeks, collars on cats reduced the flea population by 85 to 90% within 1 to 5 days following infestation. Collars on cats, as on dogs, were 100% efficacious only during the first 2 weeks of the trial.