A previous survey for glucose-6-phosphate dehydrogenase (G6PD) deficiency among the Saudi Arab population residing along the Persian Gulf perimeter revealed a high frequency of the enzyme defect among subjects from oasis areas. The investigation reported was undertaken to supplement the previous study using a screening test with the same reliability as the conventional Motulsky brilliant cresyl blue procedure, to measure enzyme activity with a colorimetric assay among a small group of Saudis, and to study the distribution of the sickling trait among the same population. The findings substantiated the earlier study, indicated a close correlation between the distribution of the sickling trait and the enzyme defect, revealed a striking association between the sickling trait and G6PD deficiency among oasis subjects, and revealed a broad range of enzyme activity among enzyme-deficient males.