Two cases are reported which illustrate important epidemiological aspects of lymphatic filariasis--prolonged longevity of the adult parasite and the possibility of transmission by individuals with ultra-low level microfilaremia. These cases demonstrate that people can remain carriers of microfilariae in the peripheral blood for many years without reinfection, and even those with a low level microfilaremia can constitute a significant reservoir of mosquito infection. Such cases represent one of the most serious obstacles to the eradication of lymphatic filariasis in regions where control is based on chemotherapy.