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A key step in the cellular differentiation of Dictyostelium is the degradation of glycogen to provide the precursors for synthesis of the structural end products of development. We have found that the enzyme that initiates this degradative pathway, glycogen phosphorylase (1,4-alpha-D-glucan:orthophosphate alpha-glucosyl-transferase, EC 2.4.1.1), is developmentally regulated and exists as two forms. During the time course of development, a previously undescribed activity, the "b" form, decreases, whereas that of the "a" form increases. The b form is inactive unless 5'-AMP is included in the reaction mixture. The mechanism of activation by 5'-AMP is by a 40-fold increase in the affinity of the phosphorylase for its substrates. Both forms were purified to homogeneity. They have identical subunit molecular weights of 90,000 and both exist as a dimer under non-denaturing conditions. The two forms are also identical with respect to salt inhibition, optimum temperature for activity, and pH optimum. They differ in their elution from DE52-cellulose, affinity constants, thermal stability, affinity for 5'-AMP-Sepharose, and their peptide maps. Attempts to demonstrate interconversion of the two activities by a kinase-directed phosphorylation have been unsuccessful. We report here on the existence, the developmental regulation, the purification to homogeneity, and some of the physical and kinetic properties of both the 5'-AMP-dependent and -independent forms of the enzyme.