Pasteurella multocida is an important bacterial pathogen responsible for zoonotic infections, avian cholera, soft tissue infections, and respiratory disease in bovines and a wide variety of other animal species. Although a variety of virulence factors have been identified from P. multocida, the role of biofilm formation in pathogenesis has not been adequately investigated. Histophilus somni is also an important bovine pathogen, and it is not unusual for multiple infectous agents (bacteria and viruses) to be isolated from and contribute to bovine repiratory disease. Using crystal violet staining, electron microscopy, and confocal laser scanning microscopy we have determined that P. multocida isolates from various animals and infectious sites are capable of forming a biofilm, but that the amount, size, and conformation of the biofilm varied widely among isolates. Most P.multocida type A and D isolates from birds and swine, respectively, formed large amounts of biofilm, whereas most bovine isolates formed little biofilm. However, bovine isolates that formed the least amount of biofilm were the most mucoid and produced the largest amont of capsular polysaccharide. A capsule-deficient P. multocida mutant of a poor biofilm-forming strain produced substantially more biofilm than the parent. Furthermore, a novel exopolysaccharide (EPS) was produced during P. multocida biofilm formation and was purified by phenol extraction, enzyme digestion, and gel filtration. The EPS was determined to be a galacto-mannan polymer by gas chromatography-mass spectrometry, and rabbit antiserum to the EPS cross-reacted with the galacto-mannan EPS present in the H. somni biofilm by ELISA. Bovine isolates that formed little biofilm independently, coexisted with and grew within a biofilm formed by H. somni. Therefore, highly encapsulated strains of P. multocida that cannot produce a good biofilm appear to form a symbiosis with H. somni to establish a polymicrobial biofilm infection.