Chrysops atlanticus, an American species of deerfly collected along the Missisippi Gulf Coast, will support the development of the microfilaria of human Loa loa to the infective stage. Development takes place in the fat body of the fly and requires 9 to 10 days in flies maintained at a temperature of 80°F and 60% to 80% relative humidity. The site of development and pattern of morphogenesis of the parasite in C. atlanticus is virtually identical to that described in natural African vectors. Chrysops atlanticus will support the development of large numbers of L. loa to the infective stage without apparent ill effects. Two uninfected patas monkeys each were given 75 third-stage larvae obtained from experimentally infected C. atlanticus. Both animals developed patent infections in approximately 5 months, clearly demonstrating that the entire life cycle of Loa loa can be maintained in the laboratory outside endemic areas.