Diuron (3-(3,4-dichlorophenyl)-1,1-dimethylurea) and antimycin, both inhibitors of mitochondrial respiration, block electron flow between cytochromes b and c1. Mutants resistant to either drug have been selected using Schizosaccharomyces pombe strains with an extrachromosomally inherited mutator. In analogy to Saccharomyces cerevisiae these mutational sites were assumed to map in the cytochrome b gene. DNA sequence analysis showed that two changes in the same nucleotide are responsible for resistance to antimycin and diuron. Analysis of resistant and sensitive progeny of crosses between the mutants and the wild type confirmed the correlation between mutational alteration and resistant phenotype.