Forty-one experimental and 37 control cats were each infected with 50 Brugia malayi larvae in such a way that a preponderance of the larvae remained localized in the popliteal lymph node or in the lymphatics of the leg draining into that node. During the 1st week after infection cats were treated with varying doses of diethylcarbamazine citrate (DEC). Two weeks after infection, necropsy for worm recovery was performed on treated and control cats. No living larva was recovered from 21 of 22 cats treated with a total of 10 mg DEC/kg body weight or greater. A single living larva was recovered in only 2 of 5 cats treated at 5 mg/kg. At 2 mg/kg, 8 of 10 cats had substantially fewer larvae than their controls; the remaining 2 were negative. In 4 cats treated with a total of 1 mg/kg, there was no reduction of larvae. All 37 untreated controls harbored living larvae, with a mean of 56% of the inoculum being recovered.