Isolates of bovine viral diarrhea virus (BVDV) were segregated into two groups based on comparison of sequences from the 5' untranslated region (UTR) of the viral genome. Phylogenic analysis suggested that these groups, termed BVDV I and BVDV II, are as different from each other as reference BVDV (BVDV-NADL, BVDV-SD-1, BVDV-Osloss) are from hog cholera virus. Polymerase chain reaction (PCR) tests, based on the 5' untranslated region and the genomic region coding for the p125 polypeptide, were designed to differentiate between BVDV I and BVDV II. Using these tests, 76 of 140 isolates of BVDV were identified as BVDV II. Antigenic and pathologic differences were noted between BVDV I and BVDV II viruses. Among BVDV I were viruses commonly used in vaccine production, diagnostic tests, and research. BVDV II was isolated predominantly from fetal bovine sera, persistently infected calves born to dams vaccinated against BVDV, and cattle that had died from an acute form of BVDV termed hemorrhagic syndrome.