The prevalence of lactase persistence is high in Saudi Arabia.To identify a DNA variant for the lactase persistence/non-persistence trait in adult Arabs in Saudi Arabia.We sequenced DNA from 432 anonymous neonatal blood donors from five different regions of Saudi Arabia to cover the 400 bp region surrounding the previously identified lactase persistence/non-persistence variant C/T-13910 residing in intron 13 of the MCM6 gene.Two anonymous blood donors carried the C/T-13910 genotype. One variant, T/G -13915, residing 5 bp upstream of the C/T-13910 variant, was present in 332 of 432 (76.9%) of the neonatal samples, compatible with previous prevalence figures of lactase persistence in urban Saudi populations. Determination of disaccharidase activities in 25 intestinal biopsy samples showed a highly significant correlation between lactase activity and the T/G-13915 genotypes (p<0.001; Fisher exact test) as well as between the L:S ratio and the aforementioned genotypes (p<0.001; Fisher exact test).The T/G-13915 variant is the founder mutation of lactase persistence in an urban Saudi population. The results obtained here have implications for genetic testing of adult-type hypolactasia and to analysis of human evolution, the origin of cattle domestication and migrations of the populations in the Arabian peninsula.