Two hundred and twenty adult males living in endemic onchocerciasis areas in Ivory Coast, with a mean of 59 to 64 mf/mg of skin microfilariae, having or not ocular lesions, were divided into 4 homogeneous groups and treated by placebo or by a single oral dose of 100, 150 or 200 mcg/kg of ivermectin. Parasitological, clinical, ophthalmological, biological data were gathered before treatment (J less than 1) and at day 4 and 3, 6, 12 months post treatment. The doses of 150 to 200 mcg/kg gave the best results with a reduction of microfilariae of 75 to 79% at day 4 and of 97 to 99% at 3 months. A slight increase appears at six months reaching to 10% of the initial level, at 12 months. These posologies reduce also the number of ocular microfilariae 12 months after treatment. The percentage of patients presenting microfilariae in the cornea varies from 4 to 14% according to the given dose whereas it was initially between 26 and 33%, and in the anterior chamber from 22 to 16% instead of 62 to 67%. In patients who were still positive after treatment the detected number of ocular microfilariae was very small. Side effects starting 12 to 24 hours after treatment are similar to those appearing during the normal evolution of onchocerciasis. They were observed in 36% of subjects receiving a placebo and 56 to 65% of treated subjects. Statistically they are neither correlated with the intensity of parasitism nor to the given posology and disappear spontaneously few days later or after administration of aspirin and/or antihistaminic. Ivermectin given at a single oral dosage of 150 to 200 mcg/kg is a powerful microfilaricidal drug with a rapid and prolonged action and without major side effects.