We have cloned and sequenced the aldA (encoding aldehyde dehydrogenase) gene of Aspergillus nidulans. The gene contains two introns which are similar in size and structure to other fungal introns. The amino acid sequence of aldehyde dehydrogenase (497 residues) shows a significant level of homology with analogous sequences in other organisms. Comparison of the primary structure of the active sites of the mammalian cytosolic and mitochondrial enzymes shows that the Aspergillus enzyme closely resembles the mammalian mitochondrial enzyme. Analysis of the 5' non-coding region of the aldA gene shows a TATA-like sequence located 90 bp upstream from the initiation codon. Two messenger-RNA start points are located 36 and 42 bp upstream from the start codon.