Dinoflagellate luciferin has been found to crossreact and emit light with euphausid photoprotein; and euphausid fluorescent substance gives luminescence with dinoflagellate luciferase. Luciferin and the fluorescent substance, both highly unstable and fluorescent compounds, are biochemically similar but not identical. Preliminary spectral and chemical data suggest that both compounds contain an open-chain polypyrrole structure, novel among compounds so far known to be involved in light emission in any biological system. The dinoflagellates and euphausids are phylogenetically distant; the possibility that the latter obtain the molecule nutritionally from the former is suggested.