Recently several enzymes which require FMN as a cofactor have been shown to be inhibited by blue light in the presence of added FMN [l-3] . Photoinactivation of the enzyme protein occurred via the formation of triplet FMN. The present paper reports, for the first time, an FMN-mediated photoinhibition of NAD-malate dehydrogenase, a key enzyme of the Krebs Cycle, at low light intensities. Evidence is presented that a binding site for FMN must exist on this enzyme.