Pasteurella and Mannheimia organisms are β-hemolytic, gram-negative, aerobic, nonmotile, nonsporeforming coccobacilli in the family Pasteurellaceae. This family tends to inhabit the mucosal surfaces of the GI, respiratory, and genital tract of mammals. Many are known as opportunistic secondary invaders. Some species show preferences for specific surfaces and hosts. Updating of phylogenetic data has resulted in renaming based on gene sequence analysis. As a result, P haemolytica biotypes A and T were reclassified as M haemolytica (biotype A) and P trehalosi (biotype T). More recently, P trehalosi has been reclassified as Bibersteinia trehalosi. Each isolate of M haemolytica and B trehalosi is designated with a biotype and serotype. M haemolytica A2 is the most common strain isolated from sheep and goat respiratory pasteurellosis, although A6, A13, and Ant have been reported in sheep and Ant in goats. M haemolytica A2 is routinely reported from cases of mastitis in sheep. B trehalosi T3, T4, T10, and T15 have been most often associated with the systemic or septicemic form of pasteurellosis affecting lambs. These serotypes have been regrouped to B trehalosi biotype 2, and a new biotype 4 has been added. B trehalosi is often isolated from the lungs of sheep, goats, and cattle, but pathogenicity is variable and may be incidental. P multocida has also been reported as a cause of pneumonic pasteurellosis in sheep and goats and has been isolated in herd outbreaks of septic arthritis. M haemolytica is the most commonly isolated bacteria in clinical cases, followed closely by B trehalosi, with P multocida seen less frequently.