The STP-C488 open reading frame of herpesvirus saimiri encodes an oncoprotein that has transforming and tumor-inducing activities independent of the rest of the herpesvirus genome. STP-C488 protein has an unusual, membrane-associated, fibrous structure and is located primarily in perinuclear compartments. We now report that STP-C488 is phosphorylated in vivo. The phosphorylated form, which accounted for about 15% of STP-C488 in transformed cells, migrated slightly more slowly through sodium dodecyl sulfate-polyacrylamide gels than unphosphorylated STP-C488. A serine residue near the amino terminus was shown to be the site of phosphorylation. However, phosphorylation was not required for transformation of Rat-1 cells by STP-C488.