Phosphorylase kinase (PhK) coordinates hormonal and neuronal signals to initiate the breakdown of glycogen. The enzyme catalyzes the phosphorylation of inactive glycogen phosphorylase b (GPb), resulting in the formation of active glycogen phosphorylase a. We present a 9.9 angstroms resolution structure of PhK heterotetramer (alphabetagammadelta)4 determined by cryo-electron microscopy single-particle reconstruction. The enzyme has a butterfly-like shape comprising two lobes with 222 symmetry. This three-dimensional structure has allowed us to dock the catalytic gamma subunit to the PhK holoenzyme at a location that is toward the ends of the lobes. We have also determined the structure of PhK decorated with GPb at 18 angstroms resolution, which shows the location of the substrate near the kinase subunit. The PhK preparation contained a number of smaller particles whose structure at 9.8 angstroms resolution was consistent with a proteolysed activated form of PhK that had lost the alpha subunits and possibly the gamma subunits.