Bacteriophages are involved in many aspects of the spread and establishment of virulence factors in Staphylococcus aureus, including the mobilization of genetic elements known as S. aureus pathogenicity islands (SaPIs), which carry genes for superantigen toxins and other virulence factors. SaPIs are packaged into phage-like transducing particles using proteins supplied by the helper phage. We have used cryo-electron microscopy and icosahedral reconstruction to determine the structures of the procapsid and the mature capsid of 80α, a bacteriophage that can mobilize several different SaPIs. The 80α capsid has T=7 icosahedral symmetry with the capsid protein organized into pentameric and hexameric clusters that interact via prominent trimeric densities. The 80α capsid protein was modeled based on the capsid protein fold of bacteriophage HK97 and fitted into the 80α reconstructions. The models show that the trivalent interactions are mediated primarily by a 22-residue β hairpin structure called the P loop that is not found in HK97. Capsid expansion is associated with a conformational switch in the spine helix that is propagated throughout the subunit, unlike the domain rotation mechanism in phage HK97 or P22.