Abstract: A vaccine and method for treating fish susceptible to infection by Renibacterium salmoninarum is described. The vaccine comprises killed microorganisms that lack intact cell-surface-associated protein p57. The vaccine can be used in combination with additional materials, such as, without limitation, adjuvants, plasticizers, pharmaceutical excipients, antigens other than the cells lacking intact cell-surface-associated protein p57, diluents, carriers, binders, lubricants, glidants, aesthetic compounds, such as flavoring and coloring agents, and combinations thereof. The vaccine may be enteric-coated for oral delivery. The enteric coating generally comprises a polymer coating that is impervious to dissolution and/or degradation in the stomach, but is dissolved upon passing to the higher pH environments of the intestine. A preferred embodiment of the vaccine is made using spherical sugar microspheres.