The flagellar accessory protein FlaH is thought to be one of the essential components of an archaeal motility system. However, to date biochemical and structural information about this protein has been limited. Here, the crystallization of FlaH from the hyperthermophilic archaeonMethanocaldococcus jannaschiiis reported. Protein crystals were obtained by the vapour-diffusion method. These crystals belonged to space groupP3₁21, with unit-cell parametersa=b= 131.42,c= 89.35 Å. The initial solution of the FlaH structure has been determined by multiple-wavelength anomalous dispersion phasing using a selenomethionine-derivatized crystal.