A series of mitochondrially inherited mutants of yeast has been analysed, which were previously identified as showing resistance to the antibiotics venturicidin or ossamycin and whose mutations showed tight linkage to oligomycin-resistance alleles affecting subunit 9 of the mitochondrial ATP synthase. DNA sequence analysis of the oli1 gene of these mutants has been used to define the nature of amino acid substitution in the subunit 9 protein. In the case of the two venturicidin-resistant mutants, mutations affect amino acids on the N-terminal stem of the protein, namely Gly25----Ser (venR ossS oliR) and Ala 27----Gly (venR ossS oliS). The mutations found in the two ossamycin-resistant mutants affect amino acids on the C-terminal stem of the protein; namely Leu53----Phe (vanS ossR oliR) and Leu57----Phe (venS ossR oliS). These results allow us to further develop a fine structure map of domains within the subunit 9 protein involved in antibiotic interaction.