A large DNA virus, designated koi herpes virus (KHV), carp interstitial nephritis gill necrosis virus (CNGV) and Cyprinid herpes virus‐3 (CyHV‐3), causes massive mortality of carp. Morphologically, the virus resembles herpes viruses, but it contains a genome of ca 295 kbp, larger than that of any Herpesviridae member. Interestingly, three CyHV‐3 genes, thymidylate monophosphate kinase (TmpK), ribonucleotide reductase and thymidine kinase, which are involved in deoxynucleotide tri‐phosphate synthesis, resemble those of pox viruses. In addition to the TmpK gene, which is nonexistent in the genome of herpes viruses, CyHV‐3 contains a B22R‐like gene, exclusively expressed by pox viruses. These results raise questions on the phylogenic origin of CyHV‐3.