The addition of glyoxal, a trapping agent, caused simultaneously the accumulation of a green fluorescent compound and the inhibition of riboflavin formation in non-growing cells of Eremothecium ashbyii. The fluorescent compound purified was identified as 8-ribityllumazine from the results of spectrophotometric and fluorometric analyses. Accordingly, the fragment, except for the glyoxal portion on the 8-ribityllumazine molecule, 4-ribitylamino-5-amino-2,6-dihydroxypyrimidine, is believed to be an intermediate in the riboflavin biosynthetic pathway.