The circovirus Beak and feather disease virus (BFDV) causes psittacine beak and feather disease (PBFD) that is characterised by a chronic disease process associated with feather abnormalities, beak deformities and eventual death in various species of birds in the order Psittaciformes. This disease is seen in captive and wild psittacine species in Australia and several other countries and is a significant threat to the survival of some endangered psittacine species. This thesis reports on genetic studies that have furthered the understanding of the diversity of BFDV present within Australia. These studies have optimised methods of detecting BFDV. They have also resulted in the production of an immunogenic and antigenic recombinant BFDV Capsid protein that could lead to alternate methods of producing viral antigen for serological tests and the development of a BFDV vaccine. To assess the optimal method of the detection of BFDV infection, feather and blood samples were submitted by referring veterinarians throughout Australia from psittacine birds tentatively diagnosed with PBFD or with a history of being in contact with PBFD-affected birds. These samples were examined by 3 procedures commonly used to detect BFDV infection: a polymerase chain reaction (PCR) assay and haemagglutination (HA) for the detection of virus, and haemagglutination inhibition (HI) tests for the detection of virus antibody in response to infection. Of the samples examined from 623 psittacine birds, the prevalence of BFDV DNA in feather samples detected by PCR was 18.85%. There was a strong correlation between PCR and HA testing of feather samples, although possible false-positive and false-negative PCR and HA results were obtained in some samples. Of the 143 birds that were PCR feather-positive only 2 had detectable HI antibody and these birds were also HA feather-negative, which suggests that they were developing immunity to recent infection. All birds with HI antibody were feather HA negative. Despite the rare occurrence of PBFD in cockatiels (Nymphicus hollandicus), 2 of the 13 samples collected from this species were PCR and HA positive indicating that this species can be infected with BFDV. Three studies were undertaken to further our understanding of the genetics of BFDV in Australian avifauna: sequence analysis of the BFDV detected in a grey cockatiel (Nymphicus hollandicus), a species normally considered resistant to infection with BFDV; analysis of the genome of BFDV present in lorikeets (Trichoglossus sp.) in Australia; and analysis of the genome of BFDV detected in endangered swift parrots (Lathamus discolor). Sequence analysis of the entire genome of the cockatiel BFDV isolate revealed that it clustered phylogenetically with 2 other viruses, one from a sulphur crested cockatoo (SCC1-AUS) and one from a Major Mitchell cockatoo (MMC-AUS), which suggests that this isolate from the grey cockatiel was not a cockatiel-specific biotype. Phylogenetic analysis of the ORF V1 of BFDV detected in 7 lorikeets demonstrated these 7 isolates clustered phylogenetically with other BFDV isolates obtained from Loriidae species elsewhere in the world and confirmed the presence of a loriid-specific genotype. Phylogenetic analysis of the sequence data generated from ORF V1 of virus detected in 2 endangered swift parrots provided evidence they were also infected with BFDV genotypes derived from other species of birds, one isolate clustering with viruses from a Loriidae genotype and the other with isolates derived from species of Cacatuidae and Psittacidae. As part of this research, a baculovirus expression system was successfully developed for the production of recombinant BFDV Capsid protein. Inoculation of this protein into chickens resulted in the development of HI antibody, which demonstrated its immunogenicity. When used as an antigen in HI tests it detected antibody in virus-infected birds, which demonstrated its antigenicity. This protein offers potential application as an antigen for the development of serological tests and as an immunogen for incorporation into vaccines for control of PBFD.