A method is presented for the separation and identification of a new N-acetyl keto derivative of fumonisin B1 (FB1) produced in solid corn culture. Cultures of Fusarium proliferatum (M-1597) were purified using preparative hplc, and the new fumonisin was detected by negative-ion esms. Structures were confirmed by 1H- and 13C-nmr spectroscopy. The new fumonisin differs from FB1 in that the tricarballylic acid functionality at the C-15 position of the eicosane backbone is replaced by a ketone and the amino group is acetylated. Direct analysis of the culture material by negative-ion electrospray lc/ms confirmed that the new fumonisin is produced naturally by the fungus.