BACKGROUND: Newcastle disease (ND) is caused by serotype I of avian paramyxoviruses. The ND virus (NDV) strains are conveniently grouped as velogenic, mesogenic, lentogenic, and nonpathogenic-intestinal pathotypes. OBJECTIVES: The purpose of this study was to determine the pathogenicity indices of the isolated NDVs from poultry flocks in Iran. METHODS: Samples were provided from poultry flocks in different provinces of Iran and prepared for NDV isolation. From many isolated NDVs, 12 isolates belonged to 10 provinces with highly populated poultry farms which were selected for this study. A clone for each of these virus isolates was generated using limiting dilution procedure. Then, the mean death time (MDT), intracerebral pathogenicity index (ICPI), and intravenous pathogenicity index (IVPI) were determined for each virus clone and compared with those of standard virulent strains such as Hertz 33.56 and Texas GB. RESULTS: The results showed that the pathogenicity indices of the NDVs in the present study ranged from 41.6 - 60 hr for MDT, 1.76 - 1.91 for ICPI, and 2.68 - 2.88 for IVPI indicate which the velogenic type of our viruses. CONCLUSIONS: The findings of this study suggested that the very virulent NDVs currently circulating in Iranian poultry flocks are close to and even more virulent than standard virulent NDVs. Isolation, identification, pathotype determination, and molecular characteriz-ation of Iranian NDVs may help authorities to make right decisions to reduce the risks posing the Iranian poultry industry.