A cyclic GMP dependent protein kinase was purified 105-fold from the pupae of Bombyx mori, by using calcium acetate precipitation, acid-treatment, ammonium sulfate fractionation, DEAE-cellulose chromatography, and Sephadex G-200 gel filtration. The purified enzyme was able to phosphorylate several proteins, among which histone F_2b was the best substrate as far as tested. The kinase was activated only by the 3', 5'-cyclic monophosphate of adenosine and guanosine, but not by their 2', 3'- isomers. The enzyme exhibited an activity to bind cyclic [3H] GMP. The rate of binding increased in parallel with the increasing amount of the enzyme protein. Both of the phosphorylation activity and the binding activity showed an optimum at pH 6.5~7.5. The binding of cyclic [^3H] GMP was inhibited by non-labeled cyclic GMP and cyclic AMP. The inhibition by the former was 100-times as large when compared to that by the latter at their concentrations between 10^-7 and 10^-4M. The protein-phosphorylation activity was inhibited by derivatives of adenine including its deoxy-compounds. The inhibition by guanine or cytosine derivatives was slight if any. The order of inhibitory capacities of adenine derivatives was ADP>adenosine> adenine>AMP. The inhibition rate did not depend on the concentration of cyclic GMP or cyclic AMP, but it decreased as the amount of ATP was increased. Double reciprocal plots of the phosphorylation rate against ATP concentration suggest that the adenine derivatives competed with ATP. On the other hand, cyclic [^3H]-GMP-binding activity did not change according to the concentration of ADP, suggesting that the cyclic GMP-binding site and the protein-phosphorylation site are separately located in the enzyme protein.