Blastomyces dermatitidis is a dimorphic fungus, growing in the environment in a mycelial form and in mammalian tissue in a yeast form. In common with other dimorphic fungi, the life cycle of B. dermatitidis is not dependent upon entering a living host and establishing infection and disease. On the contrary, infection of living beings is an accidental occurrence. One corollary of this concept is that fungal dimorphism did not evolve as a response to survival at the higher temperatures of the infected animal. While ambient environmental temperatures (specifically in the soil) may fluctuate to reach those present in potential mammalian hosts, mean mammalian body temperatures most probably exceed those in the environment, although exact data on this point are not readily available. Therefore, there must be some other explanations for the evolution of fungal dimorphism, which presently remain unknown.