Skin snips from 23,373 persons living in the Thyolo (formerly Cholo) highlands focus of southern Malawi showed infection with Onchocerca volvulus to be unevenly distributed within the highland area. In the centre of the focus most adults were infected, though intensity of infection was light, not exceeding a geometric mean of 8 microfilariae per mg of skin. It was estimated that 327,000 persons live in areas where prevalence of infection exceeds 10% and, of these, 94,500 persons over the age of one year are infected with O. volvulus. There is a significant relationship between infection with O. volvulus and bilateral blindness in the Thyolo focus, although the number of persons blind from onchocerciasis is probably less than 300.