Using a previously developed primer system, nifH gene fragments 450 nucleotides long were amplified, cloned, and sequenced for representatives of nitrogen-fixing methanotrophic bacteria of the genera Methylococcus, Methylocystis and Methylosinus. Fragments of nifH genes were also detected and sequenced in representatives of the genera Methylomonas and Methylobacter, which were not considered diazotrophs until recently. Phylogenetic analysis revealed remoteness of nifH genes sequences of methanotroph types I and II. At the same time, close relationship was found between nifH of type I methanotrophs and representatives of gamma-proteobacteria and between nifH genes of type II methanotrophs and representatives of alpha-proteobacteria. The results obtained in this study are in good accordance with the data of phylogenetic analysis based on 16S rRNA sequence comparison with the only exception of Methylococcus capsulatus strains, whose nifH genes proved to be closely related to nifH genes of Methylocystis and Methylosinus representatives. Our findings extend the database of primary sequences of nifH genes and allow the contribution of methanotrophs to the process of nitrogen fixation to be estimated.