Blastomycosis is a fungal disease caused by the dimorphic fungal organism, Blastomyces dermatidis. In the tissue or yeast form, the organisms appear as highly refractive, double walled yeast cells which vary from approximately 8-15 microns in diameter. The budding yeast cells show a very characteristic broad attachment. The mold phase is rapid growing and the hyphae branch at right angles.