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The outer surface protein C (ospC) locus of the Lyme disease bacterium, Borrelia burgdorferi, is at least an order of magnitude more variable than other genes in the species. This variation is classified into 22 ospC major groups, 15 of which are found in the northeastern United States. The frequency distributions of ospC within populations suggest that this locus is under balancing selection. In multiple-niche polymorphism, a type of balancing selection, diversity within a population can be maintained when the environment is heterogeneous and no one genotype has the highest fitness in all environments. Genetically different individuals within vertebrate species and different vertebrate species constitute diverse environments for B. burgdorferi. We examined four important host species of B. burgdorferi and found that the strains that infected each species had different sets of ospC major groups. We found no …