The prevalence of osteo-arthrosis of the hip, the knee, the small joints of the hand and the metatarsophalangeal joint of the big toe, has been studied in a South African Negro population. The pattern of joint involvement differed considerably from that expected in a White population. Interphalangeal osteo-arthrosis was much less common in Black women than in White, and Heberden's nodes were seen in only 3 of 93 respondents who were over 65 years of age. Osteo-arthrosis of the metacarpophalangeal joints also, was less common in Black women than in White, but the Black men showed a much greater involvement than that seen in White men. As in White peoples, osteo-arthrosis of the metatarsophalangeal joint of the big toe was commoner in women than in men, but the over-all prevalence did not reach the high levels in Whites. Osteo-arthrosis of the knee was much commoner in the women than in the men; this was associated with a marked tendency to obesity in the older women. The hip joint which is a common site for osteo-arthrosis in Whites, was seldom affected in this Black population. It is clear from this study that the apparent rarity of certain forms of degenerative arthritis in the South African Negro is not owing to any inherent resistance to the disease, because osteo-arthrosis of certain joints, such as the knee in women and the metacarpophalangeal joints in men, is even more common in Blacks than in Whites. The differences encountered can, in many instances, be attributed to differences in occupation and life style in the two populations; but genetic factors, may also play a part in the sense that certain predisposing disorders, such as congenital subluxation of the hip, appear to be far less common in Blacks than in Whites.