Rabbits (Oryctolagus cuniculus) of non-dwarf (Group A) and dwarf (Group B) strains showing a common clinical sign of head tilt (torticollis) were examined. With 1 exception, all rabbits of group A had otitis and empyema of either one or both middle ears. Pasteurella multocida was isolated from pus and from the nose of all but 1 of these rabbits, and on occasion was also isolated from the brain. By contrast in all dwarf rabbits the presence of the protozoan Encephalitozoon cuniculi was confirmed both histologically and serologically. This parasite did not affect the ears of the animals but rather the central nervous system. We assume that the different exposure to both agents, rather than the degree of susceptibility, was responsible for the differences found between the 2 types of rabbits.