Epizootics of Koi herpesvirus (KHV) cause mass mortalities in koi carp and common carp worldwide. We used a newly developed ''per-gill infection'' procedure with live KHV, and then conducted detailed histopathological and ultrastructural studies of KHV-infected cells including an examination of the morphology and morphogenesis of KHV. The primary target of KHV was respiratory epithelial cells of the gill lamellae, and release of virions from infected epithelial cells resulted in a systemic infection affecting the kidney, spleen, heart, brain and liver. The pathognomonic feature of infected cells was the formation of intranuclear inclusion bodies with marginal hyperchromatosis in the nucleus. Within the nucleus, assembly of capsids and nucleocapsids and an increase in filamentous nucleoproteins were evident. Enveloped nucleocapsids budded from the inner nuclear membrane into the perinuclear space. De-enveloped nucleocapsids were translocated in the cytoplasm to be embedded within inclusion bodies where tegumentation of the nucleocapsid occurred. Enveloped virions that had budded into intracytoplasmic vesicles and virions located extracellularly were composed of an electron-dense core, surrounded in turn by the capsid, the tegument and finally an envelope with projections. The morphology and morphogenesis of KHV were the same as those of viruses within the family Herpesviridae.Dis Aquat Organ. 2008 Jun 1980(1):1-11.Miyazaki T, Kuzuya Y, Yasumoto S, Yasuda M, Kobayashi T.Graduate School of Bioresources, Mie University, 1577 Kurimamachiya-cho, Tsu, Mie 514-8507, Japan. miyazaki@bio.mie-u.ac.jp