Histoplasma capsulatum var. duboisii has only recently been shown to occur naturally in soil admixed with bat guano in a bat cave. We report our observations on the in vitro interaction of Histoplasma capsulatum var. duboisii with other fungi recovered from the same habitat or other sources by employing co-culture technique and by investigating antibiosis between H. capsulatum var. duboisii and other fungi. The range of interactions varied from total overgrowth of Histoplasma by Aspergillus fumigatus and Microsporum gypseum, through partial antagonism by Lecythophora mutabilis, to mutual inhibition by Wangiella dermatitidis. Those fungi that exhibited a competitive advantage in terms of growth did not affect the viability of Histoplasma. Lysis and mycoparasitic structures were not observed in the co-cultures. However, the cultural filtrate of Chrysosporium indicum inhibited the growth of the Histoplasma strains. The results suggest the ability of H. capsulatum var. duboisii to co-exist with a variety of other potentially pathogenic and non-pathogenic fungi in an ecological niche.