Aims This study aimed to characterize the impact of lytic and temperate bacteriophages on the genetic and phenotypic diversity of Mannheimia haemolytica from feedlot cattle. Methods and Results Strictly lytic phages were not detected from bovine nasopharyngeal (n=689) or water trough (n=30) samples, but Myoviridae- or Siphoviridae-like phages were induced from 54 of 72M.haemolytica strains by mitomycin C, occasionally from the same strain. Phages with similar restriction fragment length polymorphism profiles (RFLP 70% relatedness) shared common host serotypes 1 or 2 (P<0 center dot 0001). Likewise, phages with similar RFLP tended to occur in genetically related host bacteria (7079% similarity). Host range assays showed that seven phages from host serotypes 1, 2 and 6 lysed representative strains of serotypes 1, 2 or 8. The genome of vB_MhM_1152AP from serotype 6 was found to be collinear with P2-like phage phi MhaA1-PHL101. Conclusions Prophages are a significant component of the genome of M.haemolytica and contribute significantly to host diversity. Further characterization of the role of prophage in virulence and persistence of M.haemolytica in cattle could provide insight into approaches to control this potential respiratory pathogen. Significance and Impact of the Study This study demonstrated that prophages are widespread within the genome of M.haemolytica isolates and emphasized the challenge of isolating lytic phage as a therapeutic against this pathogen.