This study examined 49 field isolates of the genus Avibacterium, with the 49 being allocated to 36 epidemiologically unrelated groups and one isolate from each group being examined in detail. In addition, six type and reference strains were investigated. Phylogenetic analysis of partially sequenced recN, rpoB, infB, pgi and sodA genes confirmed the existence of the species Avibacterium paragallinarum, while a species complex encompassing Avibacterium volantium, Avibacterium avium, Avibacterium gallinarum, Avibacterium endocarditis and Avibacterium sp. A could not be resolved. All isolates shared at least one identical sequence in one gene, indicating low diversity or horizontal gene transfer (HGT) between isolates. Such HGT between isolates of defined species and unclassified isolates combined with high sequence similarity can be explained as the result of an ongoing speciation process. The alternative explanation is that Av. volantium, Av. avium and Avibacterium sp. A were misclassified originally. Except for Av. paragallinarum, identification of species of Avibacterium seems problematic, even by DNA sequencing, as shown in the present investigation. The results indicate that Avibacterium probably contains only two or three species. Until the taxonomic revision is completed we recommend that isolates that do not fit with named species by genotype and phenotype be designated Avibacterium sp.