Specific consideration has been given in this review to the sequence of morphologic events which characterize the transformation of dimorphic pathogenic fungi from the saprophytic to the parasitic phase of growth. Three general mechanisms of conversion have been described for those dimorphic fungi which exist in the tissues of a host as blastospores: (1) The hyphal cells of the saprophytic phase swell, assume a yeast-like shape and eventually fragment to form a mass of individual blastospores. (2) The hyphal cells (terminal or interstitial) bud directly to form blastospores. (3) The hyphal cells form conidiospores in a fashion characteristic of the saprophytic phase, but these conidia produce buds which eventually separate from the parent spore and become the blastospores of the yeast phase of growth.