We have identified the etiological agent of hemorrhagic nephritis enteritis of geese (HNEG), a fatal disease of European geese. HNEG has been recognized in almost all goose breeding areas, with an epizootic pattern, and up to now, the infectious agent has remained unknown. In order to identify the causative agent, infected tissues from HNEG-affected geese were inoculated to 1-day-old goslings, which then developed clinical signs typical of HNEG. Tissue homogenates from these birds were subjected to Freon extraction followed by sucrose density gradient ultracentrifugation. The resulting main band was examined by electron microscopy and consisted of spherical, naked, papovavirus-like particles approximately 45 nm in diameter. The virus was isolated and propagated in goose kidney cell primary culture. Tissue- or culture-purified virus allowed the experimental reproduction of the disease in goslings. Random PCR amplification of viral nucleic acid produced a 1,175-bp fragment which was shown to be associated with field samples collected from geese affected by HNEG on commercial farms in France. Sequence analysis of the PCR product revealed a unique open reading frame, showing 63 to 72% amino acid similarity with the major capsid protein (VP1) of several polyomaviruses. Finally, based on phylogenetic analysis, we conclude that the causative agent of HNEG is closely related to but clearly distinct from other polyomaviruses; we thus have named this newly identified virus Goose hemorrhagic polyomavirus.