Only three of thirty microorganisms isolated from seven samples of fresh or lightly smoked sea fish muscle in various stages of decomposition reduced trimethylamine oxide to trimethylamine. This reduction is due to an enzyme, which activates trimethylamine oxide rendering it susceptible to reduction by many of the dehydrogenases of the bacterial cell. This enzyme, as it occurs in the intact cell, is apparently completely inhibited by toluene-treatment but not by cyanide. It. has not yet been obtained in cell-free state, and its substrate specificity has not been determined. Putrid fish muscle, with negligible amounts of trimethylamine, has been obtained by inoculating aseptically excised fish muscle with non-trimethylamine forming bacteria.