A collection of polymorphic DNA sequences has been used to analyse the variability of a panel of 91 Y chromosomes. Some sequences (DYZ1 and DYZ2) were highly polymorphic and allowed all of the chromosomes to be distinguished. Other sequences were less polymorphic and were used to construct a haplotype for each chromosome and to assign chromosomes to groups. The best understood of these loci could be used to construct a tree showing the evolutionary relationships between the groups of Y chromosomes. In general,chromosomes from different ethnic backgrounds belong to different groups.