Selenomethionyl proteins are now widely used in protein crystallography. It can be used as one of the heavy atom derivatives having sufficient phasing power. In addition, the positions of selenium atoms can be easily determined through the use of difference Fourier technique. Using such positions as a guide, labor for interpreting electron density maps is much reduced. Here, we report on one example of the structure determination using a selenomethionyl protein as one of the heavy atom derivatives, and give results of the analysis related to the use of selenomethionyl proteins in protein crystallography.