The African Programme for Onchocerciasis Control (APOC) has implemented a series of surveys aimed at evaluating the long-term impact of its activities. The region of Lastourville (Gabon) is one of the selected sites for this study. A total of 886 persons was examined for skin lesions, and 459 out of them participated in detailed ocular examinations. Blackflies were collected during one year and dissected. Although the focus was found to be hypoendemic (prevalence of nodules: 7.7%), the frequency of onchodermatitis was relatively high. The lesions of the anterior segment of the eye were rare, but the prevalence of optic nerve disease, and of choroido-retinal lesions reached 5.2 and 2.7%, respectively. The annual transmission potential (2,171 infective larvae per man) was high, when compared with the results recorded in the human population. This may be due to the presence of Onchocerca spp. of animal origin in the blackflies. These results indicate that in the area of Lastourville, though regarded as the main focus of onchocerciasis in Gabon, the disease is relatively mild.