Bialaphos (BA) and phosalacine (PA), which are produced by soil actinomycetes, are tripeptides and contain phosphinothricin (PT) as the N-terminal amino acid. PT is a glutamic acid analog containing a C-P-C covalent bond and inhibits glutamine synthetase. Therefore it is believed that PT is released from both BA and PA as the active substance by the action of peptidases. The difference between the two peptides is due to the kind of C-terminal amino acid, which is L-alanine for BA and L-leucine for PA. This suggested the possible occurrence of other natural peptides containing PT. So we attempted to obtain a new peptide with PT, and found a tetrapeptide named trialaphos (1). In this paper, we describe the isolation, chemical structure and biological activity of 1.