Fonsecin is isomeric with rubrofusarin, a red pigment first isolated by Ashley, Hobbs and Raistrick2 from Fusarium culmorum and Fusarium graminearum. Like these investigators, as well as Mull and Nord3, we first concluded on the basis of general properties that fonsecin was a dihydroxymethoxy-methylxanthone; recently, however, Dreyer4; Stout, Dreyer and Jensen5; and Tanaka and Tamura6 have established that rubrofusarin is a naphtho-y-pyrone and is represented by structure I. Using this new work as a guide, we have been able to show that fonsecin has structure II. O O#2 OK^JRaOl II: El = EZ = H; #2 = CH3 III: B! = j2 = #3 = H IV: J?! = H; Rz = jR3 = CH3 V: Rl = B3 = CH3; J?2 = H VI: JB1 = Bs = #3 = CH3When fonsecin was demethylated with hydriodic acid, nor-rubrofusarin (III) was obtained (analysis, identical ultra-violet and infra-red curves, na.p. and mixed melting point with authentic nor-rubrofusarin obtained from rubrofusarin from two different sources, supplied by Drs. H. Raistrick and J. H. Birkinshaw and by Dr. H. G. Stout). On methylation with diazomethane for a short time, fonsecin gives a monomethyl ether (IV) of m.p. 176 . Rubrofusarin monomethyl ether (V) is reported2'4 to have a m.p. of 203 -204 , which is the same as that reported by Lund, Robertson and Whalley7 for asperxanthone, a yellow pigment isolated from Aspergillus niger. A direct comparison of these two compounds apparently has not been made.Prolonged treatment of fonsecin with diazomethane gives a product (VI) identical in all respects with dimethyl rubrofusarin. When fonsecin monomethyl ether (IV) was oxidized with potassium permanganate8, 3,5-dimethoxyphth-alic acid was detectable by paper chromatography; this acid could not be found when rubrofusarin mono-methyl ether (V) was oxidized under the same conditions. These findings confirm previous observations4.Our yields of fonsecin (4-5 per cent of the dried mycelium) are much higher than those reported by Ashley et al.2 for rubrofusarin and by Lund et al.7 for asperxanthone. Our work makes these natural naphtbo-y-pyrones more readily available now for study.