A persistent paradigm in insect pathology is one that relates the insect host to certain genetic groups of insect-pathogenic fungi. This paradigm assumes that the genotype of an insect-pathogenic fungus coevolves with a certain taxon of insect host that it infects. The insect-pathogenic fungus Beauveria bassiana shows a wide host range and is considered to be a facultative insect pathogen. In this study, a population genetics analysis of B. bassiana from forested and agricultural habitats as well as from the Canadian Arctic showed distinct genetic groups associated with the three different habitats. Within each group, recombining population structures and clonally reproducing lineages were observed. The B. bassiana isolates were also assessed for their abilities to grow at 8, 15, 25 and 37 degrees C and for their tolerances to UV exposure. The genetic groups from the Arctic and from the forested habitats grew at lower temperatures, while the genetic group from the agricultural habitat grew at 37 degrees C and was tolerant to UV exposure. There were no clear associations between the genetic group and the ability to infect coleopteran or lepidopteran insect larvae. There is increasing evidence that such studies represent a significant paradigm shift; habitat selection, not insect host selection, drives the population structure of deuteromycetous insect-pathogenic fungi. We suggest that adaptation to a certain habitat type is an important criterion for identifying insect-pathogenic fungal strains for use in insect biocontrol efforts.