The ultrastructural morphology of the early phases of mycelium-yeast transition in Histoplasma capsulatum after a temperature shift from 25 degrees C to only 34 degrees C is described. Under this condition of lower temperature oxidative phosphorylation is not completely uncoupled and maximum production of heat shock proteins (hsp) occurs. 24 h after temperature shift more than 90% of the cells still appear vital. Alterations in the organization of the mitochondrial cristae are the only ultrastructural changes observed in these cells. In contrast, 70% of the cells degenerate 24 h after a temperature shift from 25 degrees C to 37 degrees C and in the remaining cells mitochondria are rarely observed. These observations are discussed in relation to the production of hsp, the uncoupling of oxidative phosphorylation and the virulence of different strains of H. capsulatum.

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Summary: The ultrastructural changes which occur during the first 24 h of mycelium to yeast transition have been studied in the dimorphic fungus Histoplasma capsulatum. A temperature shift controls mycelial to yeast transition. During the first 24 h respiratory rate, ATP and cytochrome concentration fall to very low levels. Ultrastructural observations showed that the plasma membrane became undulated and the cell wall lost its characteristic fibrous outer layer. At 8 h the ordered lammellar structure of the mitochondria was no longer apparent. 24 h after the temperature shift 70% of the cells were lysed. The remaining cells contained many cytoplasmic membrane structures; mitochondria were rarely observed. These changes are considered to be the morphological expression of the physiological events characteristic of stage one in mycelial to yeast transition. Zusammenfassung: Es wurden die Ultrastrukturveränderungen während der ersten 24 h des Myzel‐Hefe‐Phasenwechsels am dimorphen Pilz Histoplasma capsulatum untersucht. Dieser Phasenwechsel wird von einem Temperaturwechsel ausgelöst. Wahrend der ersten 24 h gehen Atmung, ATP und Cytochromkonzentration auf sehr niedrige Werte zurück. Ultrastruktur Untersuchungen zeigen, daß die cytoplasmatische Membran sich stark wellt und die Zellwand ihre charakteristische fibrilläre Aufienschicht verliert. Nach 8h war die geordnete Lamellenstruktur der Mito‐chondrien nicht mehr sichtbar. 24 h nach dem Temperaturwechsel waren 70% der Zellen lysiert. Die verbleibenden Zellen enthielten viele cytoplasmatische Membranstrukturen. Mitochondrien waren nur noch selten zu beobachten. Diese Veränderungen werden als morphologisches Korrelat zu den physiologischen Ereignissen während des Anfangsstadiums des Myzel Hefe‐Phasenwechsels angesehen.