DNA polymerase activities were surveyed in tumour tissue and normal tissue of cherry salmon (Oncorhynchus masou). High activity of DNA polymerase alpha was detected in the tumour tissue but not in the normal tissue. This indicates that the tumour cells replicate prosperously. Viral DNA polymerase activity was detected only in the tumour tissue, indicating that Oncorhynchus masou virus (OMV) DNA should replicate there. DNA polymerase beta activity was of same level in both tissues. This is the first evidence that herpesvirus DNA polymerase was detected in tumour tissue in association with herpesvirus.