The activity of pyriproxyfen in the blood diet was investigated for its efficacy against adult, egg, and larval stages of the cat flea, Ctenocephalides felis (Bouché). Adult fleas were housed in plastic cages and fed treated bovine blood using an artificial membrane system that allows fleas to feed ad libitum through a parafilm membrane. Control fleas received blood without pyriproxyfen. Results showed that ingested pyriproxyfen was relatively nontoxic to adult fleas over a period of 10 d at concentrations as high as 100 ppm. These findings are in sharp contrast to earlier studies that showed that residues of pyriproxyfen on filter paper or dog hair were highly toxic to adult cat fleas at concentrations as low as 12.5 ppm. Fleas obviously fed on blood containing pyriproxyfen because they produced large numbers of eggs. However, none of the eggs hatched. Also, larvae of untreated fleas failed to develop to adults when they were fed fecal blood excreted by pyriproxyfen-treated fleas. The results indicate that although ingested pyriproxyfen was relatively nontoxic to adult fleas, enough chemical was absorbed through the gut wall to cause ovisterilant activity, while the remainder was excreted.