The chemokine receptor CCR5 constitutes the major coreceptor for the macrophage-tropic strains of HIV-1. A mutant allele of the CCR5 gene called delta32 was shown to provide strong resistance to homozygotes against infection by HIV. The frequency of the delta32 allele was investigated in 2522 noninfected unrelated individuals from 16 different European populations. The delta32 allele was found in all populations studied, with a mean frequency of about 9.1%. A north-to-south gradient correlating latitude with delta32 allelic frequencies was found (r = 0.726), with highest allele frequencies in Denmark and Northern France, and the lowest allele frequencies in Corsica.