The Saccharomyces cerevisiae gene ABC1 is required for the correct functioning of the bc1 complex of the mitochondrial respiratory chain. By functional complementation of a S. cerevisiae abc1(-) mutant, we have cloned a Schizosaccharomyces pombe cDNA, whose predicted product is 50% identical to the Abc1 protein. Significant homology is also observed with bacterial, nematode, and even human amino acid sequences of unknown function, suggesting that the Abc1 protein is conserved through evolution. The cloned cDNA corresponds to a single S. pombe gene abc1Sp, located on chromosome II, expression of which is not regulated by the carbon source. Inactivation of the abc1Sp gene by homologous gene replacement causes a respiratory deficiency which is efficiently rescued by the expression of the S. cerevisiae ABC1 gene. The inactivated strain shows a drastic decrease in the bc1 complex activity. a decrease in cytochrome aa3 and a slow growth phenotype. To our knowledge, this is the first example of the inactivation of a respiratory gene in S. pombe. Our results highlight the fact that S. pombe growth is highly dependent upon respiration, and that S. pombe could represent a valuable model for studying nucleo-mitochondrial interactions in higher eukaryotes.