Infectious bursal disease virus (IBDV) is the causative agent of acute or immunosuppressive disease in chickens. Serotype 1 strains are pathogenic whereas serotype 2 strains neither cause disease nor protect against infection with the serotype 1 strains. The target organ of serotype 1 strains is the bursa Fabricii (BF). The molecular determinants of this tropism, and therefore pathogenicity, are poorly understood. IBDV is a non-enveloped icosahedral virus particle of 60 nm in diameter, which contains two genome segments of double-stranded RNA. Here, the generation of interserotypic reassortants using the reverse genetics approach is reported. The results of in vitro and in vivo investigations show that genome segment A determines the bursa tropism of IBDV, whereas segment B is involved in the efficiency of viral replication; they further indicate the significance of the interaction of the polymerase (segment B) with the structural protein VP3 (segment A) or the viral genome for efficient virus formation and replication.