Abstract 1. 1. From the dipterous Ceratitis capitata, at the pharate adult stage of development, two cyclic nucleotide-independent protein kinase activities (PKI-1 and PKI-2) have been isolated and partially characterized. 2. 2. The optimum Mg 2+ concentration and the substrate specificity for the different histone classes are similar for both activities and also similar to those described for the cAMP-dependent protein kinases from the same stage of the insect. 3. 3. From Ceratitis capitata brains a cAMP-binding protein (BP) has been isolated that inhibits the protein kinase activity of PKI-2 whereas the activity of PKI-1 was not affected. It is suggested that BP and PKI-2 correspond, respectively, to the regulatory and catalytic subunits of the cAMP-dependent protein kinase system.