Activyl is a monthly spot-on flea control for cats. The active ingredient in Activyl is indoxacarb. Indoxacarb works by bioactivation – enzymes inside the flea activate indoxacarb and release its full flea-killing power. There is no known flea resistance to indoxacarb. Activyl stops flea feeding, followed by paralysis and death after exposure to product. Controls fleas on cats in a home environment.