AbstractNucleotide substitution that has occurred in the repeated DNA base sequences of several species of galliformes since their origin has been investigated by DNA/DNA hybridization. The results of competition experiments and the thermal stabilities of interspecific DNA/DNA hybrids indicate that a considerable amount of base substitution has occurred between species within the Family Phasianidae. A specific group of genes, the ribosomal RNA cistrons, have been found to be conserved relative to the average DNA base sequences.Gallus domesticusDNA shows considerable base sequence heterogeneity with respect toPhasianus colchicusDNA and is about as divergent from the other members of the Family Phasianidae as are some species outside the family.