Larvae of the cat flea,Ctenocephalides felis(Bouché), are the target of numerous growth regulators. This study explores the development of an assay that tests the susceptibility of cat flea larvae to a wide range of compounds. Different rearing media and containment units were tested that would facilitate optimization. Larvae of various ages were compared, and 7-d-old larvae were found to be optimal because they were the most uniform in size and age and exhibited a need to feed. The assay could be used to distinguish insecticides from growth inhibitors. The insecticides chlorpyrifos and carbaryl caused 100% larval mortality in 24 h at 10 ppm, and cythioate and fipronil killed the larvae at concentrations of ≥100 ppm within 24 h. The insect growth regulators methoprene and pyriproxifen caused molt delay at concentrations of 100 ppm and bioallethrin delayed molt at 1,000 ppm. This assay can be used to identify compounds that are specific to cat flea larvae that may be useful in the control of cat flea infestations.