Abstract: The haemorrhagic disease of the grass carp is a wide-spread viral disease infingerling fishfarms, and it may seriously endanger the production of fingerling grasscarp. In order to make further studies on the causative agent of this disease, electronmicroscopic observations were performed on the artificially infected kidney. Theresults of the observations are reported in this paper. During 1976 and 1977, the ultra-thin sections of the kidney tissue of grass carpfingerlings (1.5-2.5 cm in total length) from three normal fish and seven moribundfish which had typical symptoms of the haemorrhagic disease after being exposed tothe causative agent were examined under electron microscope. Virus particles wereobserved in the kidney tissue of the infected fish; some of the particles were presentedas crystalline structures and orderly arranged. Similar particles were also found inthe head kidney. The virus particles similar to those of infected fish were not observedin the kidney tissues of normal fish. The observed virus was in the shape of spherical or hexagonal particles. 109 ofthese particles had a size ranging from 60 to 78 nm and a mean of 69 nm in diameter.They consisted of an outer coat 20 nm thick and a core with high electronic densityapproximately 32 nm in diameter. Another kind of virus particles was found in thecell nucleus and cytoplasmic inclusion bodies. These particles were devoid of an outercoat and had a size from 46-60 nm in diameter, average 52 nm. All such virus par-ticles were only observed in the hematopoietic tissue of the kidney, but have not beenobserved in the erythrocytes and granulocytes. There was also an absence of virusparticles in the epithelium of uriniferous tubule, which presented a normal appearance. As compared with CCV, VHS, IHN and Rhabdovirus of the grass carp (RVGC).so far as virus appearance, optimal temperature fur replication and symptom ofinfected fish are concerned, this virus appears to be similar to CCV. But its averagediameter is only 69 nm while that of CCV is 100 nm. Therefore, the observed virusparticles are considered to be different from all other known viruses of fish, so thatwe tentatively designate it as Herpesvirus of Grass earp (HVGC).