We isolated a human muscle type of carnitine palmitoyltransferase I (CPTI-M) genomic clone and determined its entire nucleotide sequence. By comparison of the nucleotide sequence of the genomic clone with that of cDNA, we determined the intron/exon junctions. For detection of the exon(s) in the 5'-region of the CPTI-M gene, we isolated cDNA clones corresponding to the 5'-region of its transcript by 5'-rapid amplification of cDNA ends (5'-RACE method). Results showed two alternative exons, 1A and 1B, that do not encode amino acids in the 5'-region of the human CPTI-M gene. The gene encoding human CPTI-M was found to consist of two 5'-non-coding exons, 18 coding exons and one 3'-non-coding exon spanning approximately 10 kbp. Furthermore, on analysis of the 5'-flanking region, a putative gene encoding a 'choline kinase homologue' was found to be located only about 300 bp upstream from exon 1A of the human CPTI-M gene. Comparison of the gene structure of human CPTI-M with the reported partial gene structure of human liver type CPTI (CPTI-L) showed that the intron insertion sites were completely conserved in these two genes.