The incorporation of (2RS) - [indole-2-13C, 2-15N] tryptophan into both roquefortine and oxaline, and the efficient conversion of roquefortine into oxaline, defines a biosynthetic pathway by which N-14 of oxaline is derived from N-5 of roquefortine (and thus from tryptophan) and not from an exogenous nitrogen source.