We have characterized the expression of a baculoviral gene, ptp, and determined the location of its gene product, a protein tyrosine/serine phosphatase (BV-PTP), during virus infection. Using an antibody raised to a BV-PTP fusion to glutathione S-transferase, we found that ptp was expressed as a 19 kDa polypeptide at late times during virus infection. However, we also found that BV-PTP was present in the virions of both the budded and occluded forms so that a low level of BV-PTP is also present at the beginning of the infection process. Biochemical fractionation also showed that BV-PTP was primarily localized to the cytoplasm in transfected cells but that BV-PTP was present in both the cytoplasm and the nucleus of baculovirus-infected cells. Immunoelectron microscopy revealed that BV-PTP was associated with fibrillar structures which form in both the cytoplasm and the nucleus of baculovirus-infected cells.