During the course of studying the soluble cyclic nucleotide-dependent protein kinases of a developing insect, three different enzymes were isolated. Two of these were found to be cAMP-dependent enzymes eluting from DEAE-cellulose in a manner identical with protein kinases I and II found in vertebrate muscle. The third enzyme appears to be unique. It has high affinity for either cAMP or cGMP (KA of 43 nM and 25 nM, respectively), the only cyclic nucleotide-dependent kinase described, to have this property. The enzyme has lower affinity for cIMP and cCMP (KA of 160 nM and 340 nM, respectively). Binding to cyclic nucleotide does not alter enzyme size. The KM for ATP is 86 microM, and among several types of histones tried, the slightly lysine-rich subgroup f2a was the best phosphate acceptor. Maximum activity was obtained with 1 mM Mg2+ while Mn2+ was completely ineffective. This new enzyme was purified to homogeneity on a cAMP affinity column as judged by two-dimensional electrophoresis. On the basis of molecular sieving and sodium dodecyl sulfate electrophoresis we have reached the preliminary conclusion that the native enzyme is a dimer of identical subunits with a molecular weight of 180,000. If the mammalian cAMP and cGMP enzymes are indeed homologous proteins, perhaps we have in this new kinase a species that represents a common ancestral protein.