SummaryExcept for subgroup A2and minor A, O and B alleles (Ax, O2and B(A)), which occur at low frequencies in the population, the major ABO alleles are considered to be homogeneous entities. The present study is the first demonstration of an extensive variability linked to the more common alleles of the blood A and O genes by digestion of the genomic DNA with different restriction enzymes and hybridization with a probe generated by PCR amplification of a segment of the last exon of the glycosyltransferase gene. For group A in Whites, two or three‐allele fragment length polymorphisms (RFLP) were demonstrated byHincII,TaqI andHintI; for group O two to five‐allele RFLP were detected in Blacks, Whites and Amerindians with restriction enzymesHincII,BglI,KpnI,TaqI andHintI. These polymorphisms probably originated by single‐base changes, although a length variation due to variable number of tandem repeats cannot be ruled out for some. Allelic association between the different restriction sites permitted the identification of seven O allele haplotypes in Blacks, and two in Whites and Indians. Three different haplotypes were identified for group A in Whites. The greater heterogeneity of the O allele among the Blacks as compared with Whites and Indians is similar to that observed for other gene systems. The polymorphisms of the histoblood group ABH antigen genes seem a useful tool for population genetics, phylogenetics and evolution st