The structural gene for 5-aminolevulinate (ALA) synthase has been cloned and sequenced from the filamentous fungus Aspergillus nidulans using an oligonucleotide probe based on a highly conserved-amino-acid sequence found in ALA synthase genes of a wide range of species. The cloned gene, hemA, has a 5 untranslated mRNA of 92 nucleotides (nt) and one intron (64 nt). The deduced protein sequence (648 amino acids) shows 64% identity to the yeast ALA synthase in the C-terminal region of 453 amino acids. The N-terminal region is typical of ALA synthase proteins in that the specific amino-acid sequence is not conserved but consists of a leader region rich in basic amino acids, believed to be involved in mitochondrial targeting, followed by a stretch of largely hydrophobic residues which may allow interaction with the inner mitochondrial membrane. Under the conditions used the transcription of hemA was unaffected by dextrose repression, heat shock, or oxygen levels.