Chick embryo liver mitochondrial matrix protein, 5-aminolaevulinate synthase, is synthesised initially as a larger cytosolic precursor. In this report we present the complete nucleotide sequence of a cDNA clone coding for the precursor together with corresponding confirmatory amino acid sequence of peptides derived from purified mature mitochondrial enzyme. The deduced amino acid sequence shows that the precursor consists of mature enzyme of 579 amino acids and an N-terminal extension of 56 amino acids. The latter presequence is highly basic in character as found with other mitochondrial preproteins.