We have studied both cyclic AMP phosphodiesterase and cyclic GMP phosphodiesterase in the myxomycete Physarum flavicomum. The cyclic AMP phosphodiesterase preparations were isolated from both the diploid plasmodial stage of the lifecycle and the haploid myxamoebal stage. The plasmodial enzyme was prepared from spent medium (extracellular) and also from purified nuclei. The myxamoebal enzyme was prepared from purified nuclei. Cyclic GMP phosphodiesterase activity was studied in purified nuclei isolated from the plasmodium. One unusual feature of all the enzymes from the plasmodium is extreme heat stability; they remain catalytically active even after exposure to a boiling water bath for twenty minutes. The myxamoebae enzyme lost all activity after five minutes in a boiling water bath. All four enzyme preparations gave linear product formation with time and all were inhibited by isobutyl-methyl xanthine, a potent competitive inhibitor of cyclic nucleotide phosphodiesterase
