Diuron (3-[34-dichlorophenyl]-1,1-dimethylurea), an inhibitor of mitochondrial respiration blocks the yeast respiratory chain between cytochrome b and c 1 . Diuron-resistant mutants of Saccharomyces cerevisiae have been selected and several mutations localized to the mitochondrial cytochrome b gene. The present paper identifies specific DNA base changes within the cytochrome b gene conferring diuron-resistance. DNA sequence analysis was done utilizing primer extension of crude mitochondrial RNA preparations in the presence of reverse transcriptase. Five independent diuron-resistant mutations have been sequenced.