The distribution of ABO and Rhesus blood groups, serum haptoglobin, and transferrin; red cell glucose-6-phosphate dehydrogenase and acid phosphatase; and hemoglobin was studied among the two aboriginal negroid tribes (Nuba and Fur); the Nilotic tribe; five tribes of Arab ancestory; and a mixed group of other minor tribes of Arab origin. The Nilotic and Nuba tribes were genetically quite distinct from the rest, with lower R1, R2, and r in the Rhesus system and low HbS and Gd-. The Arab tribes had a genetic structure which was intermediate between that of the original negroid population of the Sudan and the Arabs to the north. However, some of the Arab tribes had special genetical characteristics, e.g., Messeria had high TfD1; both Messeria and Hawazma had high HbS and Gd-, while GdA was higher only in the Hawazma. The Gaalin had very low HbS, Ro, GdA, and Gd-, suggestive of less negroid admixture compared to Messeria and Hawazma. The Fur, though an aboriginal negroid tribe, had genetic characteristics similar to Arabs.