The restriction endonuclease variation in the 12 kb region surrounding twelve Adh null alleles extracted from three Tasmanian populations has been compared with normal alleles from the same populations. Each of the null alleles had the same haplotype as revealed by digestions with eight hexanucleotide restriction enzymes. This haplotype also occurred in 4 of the 46 chromosomes bearing normal alleles which were tested; these four chromosomes with the null allele haplotype carried the AdhS allele. The data suggest that the Adh null alleles from geographically separate populations share a common ancestry and are derived from the same mutation in an AdhS allele.