In March 1981, a 32-year-old male naturalist involved with collecting mammals for study was found to have an unexpected eosinophilia. Serum parasitic screening done at the Centers for Disease Control showed filarial titer by indirect hemagglutination of 1:1,024 and of 1:40 by bentonite flocculation. His travel history disclosed 3 months spent in Cameroon in 1978. He also gave a history of intermittent arm swelling for at least a year. Peripheral blood, collected repeatedly at mid-day and midnight and tested for microfilariae by the Knott technique, was negative. He was begun empirically on diethylcarbamazine, and on day 11 of treatment he removed a worm, identified as a male Loa loa, from his leg. He completed treatment without difficulty and has done well.