This report documents the occurrence of a very virulent infectious bursal disease virus (vvIBDV) in Northern California commercial brown pullets. Diagnosis was made from multiple accessions from two neighboring and epidemiologically related ranches submitted to the California Animal Health and Food Safety (CAHFS) laboratory. Pullets, 11 and 14 wk of age from ranch A (rA) and ranch B (rB) respectively, were submitted from infectious bursal disease virus vaccinated flocks experiencing a drastic increase in mortality. The December 2008 outbreak resulted in 26% and 34% mortality on rA and rB respectively. Gross and histologic lesions characteristic of acute vvIBDV were observed. Gross lesions included edematous bursas, hemorrhages at the junction of the proventriculus and gizzard as well as hemorrhages on skeletal muscles. Microscopic lesions included severe lymphoid necrosis and inflammation in edematous bursas, lymphoid necrosis in thymus, spleen, Peyer's patches and cecal tonsils. Diagnosis of vvIBDV was confirmed by molecular characterization of the IBDV from bursas as well as viral pathogenicity in specific-pathogen-free birds. RT- PCR and nucleotide sequencing of the hypervariable region of the VP2 (vVP2) gene segment of the IBDV genome was performed on rA, rB and embryo passaged rA virions.The amino acids compatible with vvIBDV isolates: 222(Ala), 242(Ile), 256(Ile), 294(Ile) and 299(Ser) were reported from both ranches. In addition, nucleotide sequencing of a fragment of the VP1 gene demonstrated the viruses have the segment B genotype associated with highly pathogenic vvIBDV. Inocula of 10^5.5 50% egg infective dose of vvIBDV virus from rA and rB were introduced orally into two groups (g1 and g2 respectively) of 4 wk 2-day-old SPF leghorns. At 4 days postinoculation, there was 100% (22/22) morbidity in g1 and g2; 91% (20/22) mortality in g1; 100% (22/22) mortality for g2; 0% (0/20) morbidity and 0% (0/20) mortality was reported in the control group. This is the first occurrence of vvIBDV reported from birds in the United States.