The identity of the gene encoding the mitochondrial K(+)-activated acetaldehyde dehydrogenase (K(+)-ACDH) of Saccharomyces cerevisiae has been confirmed. The gene is situated on the right arm of chromosome XV, bears the systematic name YOR374w and the deduced product shows significant homology to other members of the S. cerevisiae aldehyde dehydrogenase (ALDH) family. YOR374w has now been assigned the gene name ALD7. The N-terminal amino acid sequences of K(+)-ACDHs purified from several diverse strains of S. cerevisiae were determined, and found to have 81-100% identity in alignments with the product of ALD7. Haploid mutants containing a deletion of ALD7 were constructed and, in these strains, the K(+)-ACDH was not detectable under any growth conditions examined. The activity of the Mg(2+)-activated acetaldehyde dehydrogenase (Mg(2+)-ACDH), encoded by ALD6, remained at wild-type levels in the mutants. Growth on glucose was not affected in the mutants lacking ALD7 (in contrast to the behaviour of ald6 mutants), whereas growth on ethanol was severely impaired. This observation, together with previous work by our group, shows that both the Mg(2+)- and K(+)-ACDHs are required for growth on ethanol, whilst only the former plays a role during growth on glucose.