The structure, function and regulation of the acetate inducible aciA gene of Aspergillus nidulans was analysed. The aciA locus was mapped to chromosome 1 at a position where no acetate inducible gene has been previously located. The nucleotide sequence of aciA was determined, the structures of two transcripts were determined and the sequences of the polypeptide products of the gene were deduced. Construction of an aciA loss-of-function mutant was achieved via insertional inactivation, but it did not reveal a phenotype for an aciA ^– strain. The larger polypeptide, AciA, was found to have a putative dinucleotide cofactor binding site. Acetate inducibility of aciA was found to be dependent on the amdA regulatory gene. Use of a 5 deletion series of an aciA-lacZ fusion and an in vivo regulatory protein binding (titration) assay allowed the region required for amdA activity to be localized to a 124 by segment 5 to aciA. A 13 by region of sequence similarity was observed between aciA and the coregulated amdS gene in the regions required for amdA-mediated regulation of these genes. This sequence may have a role in amdA regulation of the two genes.