In this paper we describe a novel 19 kDa sarcomeric protein named telethonin. The cDNA sequence discloses an open reading frame of 167 amino acids that does not resemble any known protein. Antibodies against a recombinant telethonin fragment were used for Western blot analysis, confirming the presence of this 19 kDa protein in heart and skeletal muscle and revealing an immunofluorescence pattern typical of sarcomeric proteins, overlapping myosin. The frequency of specific cDNA clones in different libraries indicates that the telethonin transcript is amongst the most abundant in skeletal muscle. In human, telethonin maps at 17q12, adjacent to the phenylethanolamine N-methyltransferase gene.