Immunological relationships between genera and species of microsporidia were examined by immunoblot analysis. Exospore polypeptides from 2 Nosema spp., 3 Vairimorpha spp., and 2 undescribed Vairimorpha-like spp. were analyzed. Gel electrophoresis and immunoblot analysis revealed that a variety of polypeptides, most between 15,000 and 90,000 molecular weight, were present on the exospore. The 3 Vairimorpha spp. were highly related immunologically to each other, but less so to the 2 Vairimorphalike undescribed spp. The 2 Nosema spp. immunologically distant from each other and from the Vairimorpha spp. Indirect evidence, however, indicated that many internal spore polypeptides present in both genera are similar. Cross-reactivity between exospore polypeptides from entomophilic microsporidia and antisera to a mammalian microsporidium, Encephalitozoon cuniculi was very limited.