A new compound, possessing an unusual 4H-imidazole ring was isolated; structure elucidation was performed using spectral analysis and confirmed by synthesis. Only few representatives of unsubstituted 4H-imidazoles are known (1) and to our best knowledge this paper is the first report of an 4H-imidazol-4-one as a natural as well as a synthetic product. 3-Indolyl-4H-imidazol-4-one was isolated from Dendrodoa grossularia, a Tunicate collected in Brittany, which already provided some interesting indole derivatives (2-4).