The carcass of an adult male harbor porpoise was found on the New Jersey coast. At necropsy, a plaque-like thickening of the surface of the penis was noted. Histologically, the penile lesion was characterized as a focally extensive zone of epidermal hyperplasia with a superficial dermal infiltrate of lymphocytes and plasma cells. Within the superficial epidermis, the epithelial cells were enlarged, some were necrotic and many contained amphophilic to eosinophilic intranuclear inclusion bodies. Some inclusions filled the nucleus while others were separated from the nuclear margin by an unstained zone. Electron microscopic examination revealed that the intranuclear inclusions were composed of numerous non-enveloped viral nucleocapsids that were 90-110 nm in diameter. The intercellular spaces contained many similar viral particles that were surrounded by moderately electron-dense envelopes. The enveloped viral particles measured about 180-220 nm An immunohistochemical stain for herpes simplex 1 and 2 stained the inclusion bodies. Light microscopic, electron microscopic and immunohistochemical findings are consistent with herpesvirus infection. In other host species, genital herpesvirus infections may cause severe keratoconjunctivitis, dermatitis, encephalitis, abortion, and visceral and disseminated disease. There is a single previous report of herpetic encephalitis in a harbor porpoise. The significance of this infectious disease for harbor porpoise populations is undetermined.