The pathogenicity and genetic characterizations of six Newcastle disease virus (NDV) isolates obtained from chicken farms in six different regions in Iran were carried out using conventional and molecular techniques. Based on the pathogenicity indices (MDT, ICPI, and IVPI), all of these isolates were found to be velogenic (highly virulent) strains. A sequence analysis of the full-length mRNA encoding the fusion glycoprotein precursor (F0) of the NDV's fusion proteins F1 and F2 in these six isolates showed the presence of point mutations in form of nucleic acid substitutions at positions 82((C→T)), 83((T→C)), 736((A→G)), and 1,633((G→A)). However, the nucleic acid residues at positions 330-347 of the precursor F0 gene, corresponding to the cleavage site of the F0 protein, were found to have remained conserved among the six NDV isolates. A phylogenetic comparison between the six Iranian isolates and the NDVs whose F0 gene sequences were previously deposited in GenBank Database showed that all of the newly characterized Iranian NDV isolates belonged to genotype VII.