Herpesviruses were found associated with epidermal hyperplasia of koi carp (Cyprinus carpio) in northern California (USA). Papillomas were found principally on the caudal regions of the fish including the fins. The grow ths occurred most commonly in the fall and winter among populations of captive carp. Infected epidermal cells were characterized by greatly enlarged nuclei depleted of chromatin but with thickened nuclear membranes. Numerous virions were detected in infected cells. Herpesvirus nucleocapsids in the cell nucleus had a diameter of 109 nm. Virions with envelopes with a diameter of 157 nm were abundant in cytoplasmic vacuoles. The characteristics of the papillomatous growths and the viruses were consistent with descriptions of Herpesvirus cyprini known in koi carp populations in Japan and extends the range of this pathogen to koi carp to North America.