P protein, the structural phosphoprotein of the Long strain of respiratory syncytial (RS) virus, is phosphorylated at serine residues. Some of these residues are candidates for modification by casein kinase II, as they are contained in consensus sequences. A cellular protein kinase, able to phosphorylate the P protein in vitro and apparently associated with purified RS virions, has been partially purified from HEp-2 cells. It shows several characteristics similar to those of casein kinase II. The P protein is modified in vitro by this activity mainly at serine residues located near the C terminus, which are also modified during virus infection. Thus, the P protein is phosphorylated in vivo in two regions, a central region as previously described, and another located in the C-terminal part of the molecule. The protein kinase involved in the phosphorylation of the C-terminal domain is similar to a cellular casein kinase II.