delta-Aminolaevulinate synthase (delta-ALAS) is the first enzyme in the haem biosynthetic pathway, catalysing the condensation of succinyl CoA and glycine to form delta-aminolaevulinic acid (delta-ALA). The enzyme delta-ALAS has been studied in a number of species including yeast, a facultative anaerobe, and multicellular eukaryotes. The primary structure of the protein from a number of species exhibit regions of highly conserved amino acids, and a nucleotide probe based on one region was designed and used to clone the structural gene for delta-ALAS from the filamentous fungus Aspergillus nidulans. The cloned gene, hemA, was sequenced and found to possess a single intron, of 64bp, located between 355 and 419nt in the gene; between amino acids 119 and 120 of the protein. The deduced protein sequence, of 648 amino acids, shows 64% identity over the carboxyl domain of the protein derived from the yeast HEM1 gene. The amino-terminal sequence consists of basic amino acids, believed to be involved in mitochondrial targeting, and is consistent with observations on other delta-ALAS proteins. One copy of the hemA gene was disrupted in a diploid strain of A. nidulans following the transformation with a suitable disruption vector. The recessive hemA-mutant strain was isolated by mitotic haploidisation and shown to have an absolute requirement for delta-ALA, not replaceable by haem supplied in the medium. Genetic analysis by mitotic haploidisation of a heterozygous diploid constructed between a hemA-mutant and a suitable mapping strain, located the hemA gene to chromosome VII of A. nidulans.