The NADH and NADPH specific FMN oxidoreductases from Beneckea harveyi have been purified to homogeneity as judged by single bands on sodium dodecyl sulfate gel electrophoresis. The overall purification for the NADH specific enzyme is 3000-fold and 4000-fold for the NADPH specific enzyme from a crude extract. The final step in the purification procedure is chromatography on a 5'-AMP-Sepharose 4B affinity column which results in approximately a 50-fold purification to a final specific activity of 31 mumol of NADH oxidized min-1 (mg of protein)-1 for the NADH specific FMN reductase. The NADPH specific reductase has been purified to a final specific activity of 51 mumol of NADPH oxidized min-1 (mg of protein)-1 using a NADP agarose affinity column, which results in a 70-fold purification. Molecular weights of 30 000 and 40 000 and Km's of 4.75 X 10(-5) M NADH and 4.0 X 10(-5) M NADPH have been determined for the pure NADH and NADPH specific FMN reductases, respectively. The NADPH specific FMN reductase does not utilize NADH, while the NADH specific enzyme does dehydrogenate NADPH with a maximal velocity one-tenth of that for NADH. Separate NADH and NADPH specific FMN reductases from Photobacterium fischeri could not be demonstrated.