By means of confocal and electron microscope immunocytochemistry we have studied the localization of a recently described structural protein (pUL25) of human cytomegalovirus, in both infected cells and in cells transiently transfected with UL25. pUL25 localization in infected cells was observed in typical cytoplasmic structures characterized by a very electrondense texture previously reported to accumulate other tegument proteins. At the virion level pUL25 seems to localize at the interface between the tegument and the capsid of both intracytoplasmic and extracellular virions. In UL-25-transfected cells, pUL25 has been found in characteristic para-crystalline cytoplasmic aggregates, suggesting its intrinsic ability to aggregate in a regular subunit pattern.