Experiments were carried out with Laboulbenia slackensis (Ascomycetes, Laboulbeniales), an ectoparasitic univorous fungus of Pogonus chalceus (Coleoptera, Carabidae). Relative growth of Laboulbenia slackensis populations was significantly affected by environmental conditions, i.e., the soil composition, under which the typical host was reared. Optimal conditions for the development of the fungus corresponded with the optimal habitat selected by the typical host, i.e., pure salt marsh clay. Artificial infections with L. slackensis showed that the fungus was potentially plurivorous on Carabidae and that its specificity was not fully accounted for by the host physiology or integumental characteristics. L. slackensis was successfully transferred to 19 atypical carabid host species, and the morphological characteristics used to differentiate it from related taxa are stable on these hosts. As with the typical host, successful establishment of L. slackensis on an atypical host also depended on the soil composition trader which the atypical host was reared. The host was essential, but alone not sufficient, for the establishment of the fungus. It is postulated that the specificity of L. slackensis observed in nature depends both on its host specificity and environmental preferences. Specialization of L. slackensis probably was the result of reinforced ecological isolation caused by obligate ectoparasitism, host specificity, and dependence on a specific environment that was rigorously selected by a very restricted number of host species. Host populations of univorous Laboulbeniales probably are similar to islands in the model of island biogeography.