Onchocerciasis is a major cause of blindness worldwide, and much of the blindness is caused by onchocercal chorioretinitis. In an experimental animal model for ocular onchocerciasis, intravitreal injections of 10,000 live Onchocerca volvulus microfilariae isolated from infected humans into the eyes of cynomolgus monkeys (Macaca fascicularis) resulted in patchy, progressive loss of retinal pigment with pigment clumping. Areas of pigment loss were less extensive in animals that had been sensitized with microfilariae. Intravitreal injections of dead O. volvulus microfilariae resulted in mild vitritis with relatively less clinical change noted in the retina and choroid. Histopathologic examination revealed thinning and loss of outer retinal layers with pigment migration into the retina, and inflammation was more pronounced in eyes that received live microfilariae. Clinical changes appeared in eyes receiving live microfilariae before the development of significant antibody or cell-mediated immune responses. O. volvulus microfilariae appear to be more suitable than O. lienalis microfilariae in producing lesions which resemble human onchocerciasis in the primate model.