The sequence of the small Hind III fragment F of rat-liver mitochondrial DNA (mtDNA) type A and type B was determined in order to investigate the nature of the differences between the two types of mtDNA and to determine its position in the Hind III fragment map. 2. The three differences found were point mutations. No deletion or insertion and no modification was observed. Two of the three differences affect the sequences which are recognition sites for Eco RI, Alu I and Taq I in type A, but not in type B, mtDNA. 3. The presence of an Eco RI restriction site only within the Hind III fragment F of type A shows that the fragment is situated in between the Hind III fragments A and E. 4. In one of the six reading frames, the Hind III fragment F contains the code for a carboxyl-terminal end of a polypeptide in which the three mutations do not lead to alterations in the possible aminoacid sequence. 5. The restriction sites for Taq I and a number of the sites for Alu I and Hae III were mapped. 6. The positions of the Hap II fragment J, and of a Hind III fragment G on the mtDNA were determined.
