In luminous bacteria NAD(P)H:flavin-oxidoreductases LuxG and Fre, there are homologous enzymes that could provide a luciferase with reduced flavin. Although Fre functions as a housekeeping enzyme, LuxG appears to be a source of reduced flavin for bioluminescence as it is transcribed together with luciferase. This study is aimed at providing the basic conception of Fre and LuxG evolution and revealing the peculiarities of the active site structure resulted from a functional variation within the oxidoreductase family. A phylogenetic analysis has demonstrated that Fre and LuxG oxidoreductases have evolved separately after the gene duplication event, and consequently, they have acquired changes in the conservation of functionally related sites. Namely, different evolutionary rates have been observed at the site responsible for specificity to flavin substrate (Arg 46). Also, Tyr 72 forming a part of a mobile loop involved in FAD binding has been found to be conserved among Fre in contrast to LuxG oxidoreductases. The conservation of different amino acid types in NAD(P)H binding site has been defined for Fre (arginine) and LuxG (proline) oxidoreductases.