Phosphorylase b was purified to homogeneity from the muscle of a hagfish (Paramyxine atami), as judged by electrophoretic and immunological criteria. The purified enzyme was partially but not fully activated by AMP, and its conversion into the a form resulted in a three-fold increase in activity. The enzyme was stimulated by SO4(2-), and kinetic experiments showed that SO4(2-) markedly increased the affinity of enzyme toward substrates: in the presence and absence of 0.35 M SO4(2-), the apparent Km values of hagfish phosphorylase b were 0.04 and 1.3% for glycogen, 8.7 and 66 mM for glucose 1-phosphate, and 0.05 and 1.0 mM for AMP, respectively. Electrophoretic and immunological data indicated that the hagfish possessed a single molecular form of phosphorylase, like the lamprey. Some immunological relatedness between the hagfish enzyme and the enzyme from lamprey or skate was demonstrated.