Candida aibicans C316, maintained in the yeast form, showed a proliferation of peroxisomes when grown on triolein or serum as sole carbon source but these structures were absent from glucose-grown cells. Peroxisomes were also apparent in C. aibicans obtained after injection into mice and recovery from intraperitoneal washings and kidneys; they may therefore be useful markers to assess a potential in vivo response in cells that are growing in vitro. Transcell-wall structures also occurred in C. aibicans grown on triolein or serum, and in cells cultured in wiwo, but were not seen in cells grown on glucose. These structures consisted of electron-dense f ibrillar material penetrating through the cell wall from the plasmalemma side and protruded out to the exterior of the cell. Endoplasmic reticulum, located at the periphery of the cell, was found to be in close proximity with these cell wall structures. Carnitine acetyltransferase (CAT; EC 2.3.1.7), the key enzyme for the translocation of acetyl units between intracellular compartments, was present in low activities in glucose-grown cells; its activity was increased some 100-fold in trioleingrown cells but only b o l d in serum-grown cells. It was not possible to assess this activity in the in wivo-cultured cells. Two separate CAT proteins, partially purifed from isolated microchondria and peroxisomes, respectively, were identified, with different specificities and kinetic properties.