Viral methyltranferases (MTase) are involved in the third step of the mRNA-capping process, transferring a methyl group from S-adenosyl-L-methionine (SAM) to the capped mRNA. MTases are classified into two groups: (guanine-N7)-methyltransferases (N7MTases), which add a methyl group onto the N7 atom of guanine, and (nucleoside-2'-O-)-methyltransferases (2'OMTases), which add a methyl group to a ribose hydroxyl. The MTases of two flaviviruses, Meaban and Yokose viruses, have been overexpressed, purified and crystallized in complex with SAM. Characterization of the crystals together with details of preliminary X-ray diffraction data collection (at 2.8 and 2.7 angstroms resolution, respectively) are reported here. The sequence homology relative to Dengue virus 2'OMTase and the structural conservation of specific residues in the putative active sites suggest that both enzymes belong to the 2'OMTase subgroup.