Recently, a novel pterin has been isolated from Methanogenium tationis. This pterin derivative, which was called tatiopterin, was characterized as a methanopterin-like structure with an additional aspartyl and glutamyl group in the side chain and with a 7-proton instead of a 7-methyl group in the pterin moiety. The sequence of the aspartyl and glutamyl group remained unsolved. In this study, a novel pterin was purified from Mg.tationis and analyzed by 600 MHz 1H-NMR spectroscopy and fast atom bombardment-mass spectroscopy. This pterin was found to be an aspartyl derivative of methanopterin with a 7-proton in the pterin part of the molecule. No glutamyl group could be detected. Apparently, Mg.tationis is able to synthesize two types of tatiopterin derivatives. For these cofactors the trivial names 'tatiopterin-0' (lacking a glutamyl group) and 'tatiopterin-I' (containing one glutamyl group) are introduced here.