Sickle cell anemia is common in Africa, because it is selected by the pressure related to infection with Plasmodium spp. However, its distribution varies locally, especially in isolated populations. The aim of this study was to analyze the frequency of the HbS mutation in two local tribes of Arab origin, the Shagia and Manasir, which are both from the region of the 4^th Nile cataract. The Shagia cohort investigated represents an isolated homogenous population and was relocated recently owing to construction of a dam; the Manasir cohort had significant admixtures from other tribes. Anthropological data and buccal swabs were collected from 126 Shagia and 90 Manasir representatives from the area and were analyzed using a PCR/RFLP assay specific for HbS, with confirmation by sequencing. The S mutation in the HBB gene was not detected among the Shagia and Manasir individuals investigated. The HbS AA genotype and A allele frequency therefore showed a prevalence of 100% in both groups. Lack of HbS mutation of the HBB gene in the previously unstudied Shagia group confirms that the frequency of the sickle cell gene in Sudan is tending to decrease in a northerly direction.