Highly purified Sendai virus contained a protein kinase activity which atatlysed the phosphorylation of endogenous polypeptides or exogenous protamine sulphate. The virus contained very low levels of phosphoprotein phosphatase activity. Polyacrylamide gel analysis of the reaction product indicated that the phosphorylation was specific for certain polypeptides and varied according to whether the virus was grown in eggs or in tissue culture. This variation was partially associated with the difference in the polypeptide pattern that occurred when the virus was grown in eggs or in tissue culture. Characterization of these phosphoproteins demonstrated that the phosphate was incorporated predominantly in a phosphoester linkage with theonine residues. Using a detergent and high salt solubilization procedure, the protein kinase activity was found associated within glycoprotein free virus particles but not with the nucleocapsid-associated polypeptides. In vivo phosphorylation occurred when Sendai virus was grown in eggs or in tissue culture with [32P] and the phosphorylated polypeptides were similar to those of the protein kinase reaction product. Phosphorylation could also be detected in the infected cell and could occur once the virus particle polypeptides were being synthesized. The non-structural polypeptides were not phosphorylated.