1,416 males and 564 female subjects from four Negroid and five Arab tribes and a group of mixed tribes of the Sudan were investigated for the phenotypic distribution of red cell glucose-6-phosphate dehydrogenase by starch gel electrophoresis. In general, the tribes of Negroid origin had higher frequency of GdA compared to the tribes of Arab ancestry. However, the Nilotes showed a lower frequency of GdA allele and the Mahass tribe claiming an Arab origin had a higher frequency of GdA. The immigrant groups from the neighbouring African countries also had a higher frequency of GdA. GdB (Khartoum) was present in low frequencies in both the Arab and Negroid tribes. A great deal of intratribal variation in the phenotypic distribution of G6PD was observed in the Nuba and Gáali tribes from different localitie