A topical (micro-droplet) bioassay is described for the cat flea, Ctenocephalides felis (Bouché). Key parameters are the short carbon dioxide anesthetization and the use of 0.1 ml of acetone per flea. The method was used to compare the effectiveness of 13 insecticides. LD₉₅ values in nanograms per flea were nitenpyram 0.68, fipronil 0.69, deltamethrin 0.70, imidacloprid 0.81, cypermethrin 5.4, fenthion 8.0, diazinon 12, permethrin 19, malathion 29, bendiocarb 170, DDT 710, propoxur 1,300, carbaryl >10,000. Experiments repeated 2–5 yr later, with five of the chemicals, in a different facility, showed only small shifts in potency (0.38- to twofold of the original LD₅₀ values).