Using a combination of PCR screening of cDNA libraries and reverse transcription PCR, we have cloned three overlapping DNA fragments that encode human carnitine acetyltransferase (CAT), a key enzyme for metabolic pathways involved with the control of the acyl-CoA/CoA ratio in mitochondria, peroxisomes, and endoplasmic reticulum. The resulting cDNA (2436 bp) hybridizes to a mRNA species of approximately 2.9 kb that is particularly abundant in skeletal muscle and encodes a 68-kDa protein containing a peroxisomal targeting signal. The sequence matches those of several tryptic peptides obtained from purified human liver CAT and shows striking similarities with other members of the carnitine/choline acetyltransferase family very distant throughout evolution. CAT cDNA has also been used for fluorescence in situ hybridization on metaphase spreads of human chromosomes, and the corresponding gene, CAT1, has been mapped to chromosome 9q34.1.