Significance The observation of dynamical diffraction in protein crystals is an interesting topic because dynamical diffraction typically occurs in high-quality crystals such as those of silicon. However, to our knowledge, a report of protein crystals showing clear dynamical diffraction does not exist. Here, we present an observation of the oscillatory profile of rocking curves for protein crystals such as glucose isomerase. The oscillatory profiles are in good agreement with those predicted using the dynamical theory of diffraction. It is demonstrated that dynamical diffraction occurs in protein crystals as well. These results indicate the need for a dynamical diffraction model in protein-crystal structure analysis and model refinement that, to our knowledge, has never been used in conventional structural analysis.