There are few studies about products of marine microorganisms compared with terrestrial ones1*. Marine microorganisms live in quite different environment from terrestrial ones, and have been expected to produce compounds which possess unique structures and activities. The investigations of metabolites of marine bacteria should be one of the important fields of natural products studies. In the course of screening for new antibiotics from marine bacteria, we found that Vibrio gazogenes ATCC29988 produced a component of magnesidin, and named magnesidin A. In this communication, we will describe the isolation, identification by mainly NMR data, determination of unclarified stereochemistry, and tautomerism of magnesidin A.