Sequences of exons 6 and 7 of the O allele of the ABO gene were studied in 317 individuals of the O phenotype from five different ethnic groups (Basques, Berbers, Akans from the Ivory Coast, and Amerindians: Cayapas from Ecuador and Aymaras from Bolivia). Twenty-one O alleles were characterized, among which 9 differed from all O alleles reported to date. The nine alleles differed from either the O01 allele (four out of nine) or O02 allele (five out of nine) by one to three point mutations. The number of different O alleles in population samples varied greatly: the highest number (13) was observed in Akans, and the lowest (5) in Amerindians. Some rare alleles previously reported by others at low frequencies were found with high frequencies in the Akans. The results also revealed a decreasing frequency of Ov7 alleles from south to north (Akans, Berbers, Basques). Berbers and Basques share two rare alleles, Ov6 and O03, which were not encountered in the other populations studied here.