Summary. In the early summer of 2014, mass mortality of Sichel (Pelecus cultratus) was 34 observed in Lake Balaton, Hungary. Histological examination revealed degenerative changes 35 within the tubular epithelium, mainly in the distal tubules and collecting ducts in the kidneys 36 and multifocal vacuolisation in the brain stem and cerebellum. The routine molecular 37 investigations showed the presence of the DNA of an unknown alloherpesvirus in some 38 specimens. Subsequently, three genes were amplified and sequenced partially from the 39 putative herpesviral genome (DNA polymerase, terminase, and helicase). Phylogenetic tree 40 reconstruction, based on the concatenated sequence of these three conserved genes, implied 41 that the virus undoubtedly belongs to the genus Cyprinivirus within the family 42Alloherpesviridae. The sequences of the Sichel herpesvirus differ markedly from those of the 43 three known cypriniviruses (CyHV-1, CyHV-2 and CyHV-3); putatively representing the 44 fourth virus species in the genus. (CyHV-2), originally described as goldfish haematopoietic necrosis virus, was isolated also in 76Japan from goldfish (Carassius auratus) [15]. Cyprinid herpesvirus 3 (CyHV-3), is also 77 known as koi herpesvirus, was described from common and koi carp [13]. The complete 78 genome sequence of the above mentioned cypriniviruses and that of the Anguillid herpesvirus 79 1 (AngHV-1) was determined [1, 5, 24]. 80During the early summer of 2014, mass mortality occurred among Sichel in Lake 81Balaton (Hungary), approximately 20.000 cadavers were collected by fishermen, other species 82were not affected. The present study was aimed at genetically characterizing a novel 83 alloherpesvirus (AHV) detected in Sichel. 84Cadavers of six Sichel were sent to our laboratory (64.5-149.7 g) for histopathological 85 and molecular investigations. All cadavers were necropsied immediately after arrival with 86 routine tissue collection from the main organ systems (gills, brain, liver, kidney, spleen, 87 intestine and eyes) for histopathological examination. Tissues were fixed in 10% neutral 88 buffered formalin, routinely processed, embedded in paraffin, sectioned at approximately 5 89 micrometer, mounted on glass slides, and stained with hematoxilin and eosin. The exterior 90 mucus, gills and eyes were sampled for parasite examination through an optical microscope. 91For bacteria isolation, kidney tissue was inoculated onto blood agar plates. 92For the molecular investigations organ samples were homogenized using 93 the TissueLyser high-throughput disruption instrument (Qiagen, Hilden, Germany) according 94 to the manufacturer's recommendations. 200 µL of supernatants from organ homogenates 95 were extracted using the Roche MagNA Pure LC automated system with a Total Nucleic Acid 96Isolation Kit (Roche Diagnostics, Indianapolis, IN) and eluted in 100 µL of elution buffer. 97Subsequently, the samples were tested for the presence of CyHV-3 with an expansively used 98 PCR for the detection of cyprinid HVs [10]. 99 4 After real...