Activyl for Cats is an inventive product from Merck, a once-a-month spot-on topical solution for total flea control in cats. It kills adult fleas and stops flea larvae and eggs from developing, thereby breaking the fleaÕs lifecycle. Activyl spreads through your pet's skin and when a flea lands on your pet, it takes up the active ingredient and causes the enzymes in the flea to change. The flea becomes paralyzed and unable to feed and ultimately dies. Product Specifications Activyl for Cats is an OTC product available in two packs: for cats and kittens weighing 2-9 lbs in 0.51 ml (0.02 fl oz) applicators and for cats over 9 lbs in 1.03 ml (0.03 fl oz) applicators.