Isolation of very virulent Marek's disease virus (vvMDV) was carried out by inoculating blood and spleen samples of Marek's disease (MD) chickens onto chicken embryo fibroblast cell cultures and into the peritoneal cavity of one-day-old chicks. Seven isolates of MDV were isolated in this experiment. By the results of cytopathic effect and fluorescent antibody tests, the 7 isolates were divided into 3 serotypes: serotype 1 of strains LTB-1, -2, -3, LTS-1, and -2; serotype 2 of strain LTB-4; and serotype 3 of strain LTB-5. Strain LTB-1 was used to inoculate experimental chickens of both layers and broilers. Early phase mortality and markedly atrophied lymphoid organs were observed during 0-2nd week post-inoculation (PI) and at 5th week PI, respectively, but were not found in uninoculated chickens. In addition, experimental chickens vaccinated with (HVT) vaccine at 1 day old and challenged with strain LTB-1 at 10th day post-vaccination, revealed early phase death before 2nd week post-challenged (PC), and lymphomatous lesions before 10th week PC. From these results of this in vivo experiment, strain LTB-1 showed the same pathogenicity as that of vvMDV, and this is the first report of vvMDV being isolated and identified in Taiwan.