Five field isolates of infectious bursal disease virus (IBDV) were isolated from bursae of sick young chickens by inoculating chicken embryo fibroblasts (CEF) while the morphological and physiochemical properities of these isolates were detected. Subtype of the virus isolates were analysed by cross virus neutralization. Virulence of IBDV isolates was determined with specific pathogen-free (SPF) chicken inoculation. Nucleotide sequence of genomic segment A of the isolate NB was further sequenced and analysed. All bursal suspensions produced cytopathic effects (CPE) in CEF after 2~14 passages. The physiochemical and morphological properties of IBDV isolates were in accordance with that of typical IBDV. The three CEF-adapted virus isolates JD1, JD2 and NB were divided into 3 subtypes of serotype Ⅰ IBDV except IBDV isolates HZ1 and HZ2 belonged to classical IBDV. In a pathogenicity experiment, the clinical signs and bursal atrophy resembling those of field outbreaks were produced and microscopic bursa lesions revealed that there was degeneration, necrosis and disappearance of lymphocytes in the medullary area of bursal follicles. Sequence data demonstrated that genomic A-segment of the isolate NB was composed of 3 259 nucleotides, encoding VP5 of 145 amino acid residues and the polyprotein (VP243) of 1 012 amino acid residues. Compared with serotype Ⅰ IBDV strains from GenBank, within serotype Ⅰ IBDV strains, the isolate NB has the highest identity to the polyprotein of the isolate JD1 (99.5%), VP2 of the isolates JD1, CEF94 and D78 (99.8%), VP3 of the isolate JD1 (99.2%), VP4 of the isolate JD1 (100%) and VP5 of the isolates JD1, HZ2, P2, CEF94, CT, Cu-1 and D78 (99.3%). In the VP2-predicted amino acid sequence of the isolate NB, amino acid residues at positions 253, 280 and 284 were consistent with and other published variant and classical IBDV strains, and different from very virulent IBDV. These results indicated that antigenic epitopes of IBDV are conformational and subtype forming of IBDV isolates originated from classical IBDV attenuated vaccine strains.