Cyromazine (2-cyclopropylamino-4,6-diamino- s -triazine) is a novel insect growth regulator that is a potent inhibitor of dipterous larvae. Cyromazine is also an extremely effective inhibitor of larval and pupal development of the dog flea, Ctenocephalides canis (Curtis). When mixed into a larval rearing medium, the LC50 and LC95 for inhibition of adult emergence were 2.11 ± 0.10 and 4.39 ± 0.23 ppm, respectively. A novel bioassay for evaluation of potential flea larvicides is described.