Koi herpesvirus (KHV), a highly contagious and lethal virus that affects both koi (Cyprinus carpio koi) and common carp (Cyprinus carpio), was isolated in 1998 from two outbreaks of koi suffering mass mortality in New York State, USA, and in Israel. The disease had been described as early as 1996 in Europe. In July 2004, this virus was found associated with a mass mortality event in wild common carp in the Chadakoin River, New York, USA (42 degree 07'N, 79 degree W). Affected fish typically showed marked hyperplasia of gill tissues, abdominal adhesions, and severe multifocal to diffuse external hemorrhages. The virus isolated in this outbreak was somewhat unusual in that it initially replicated well in fathead minnow cell cultures, which is typical of spring viremia of carp virus. Testing at the National Veterinary Services Laboratories, Ames, Iowa, USA, confirmed the virus's identity to be KHV. Koi herpesvirus is not currently on the OIE (World Organisation for Animal Health) list of notifiable diseases; however, it is capable of causing mass mortality in susceptible fish at permissive temperatures.