The ultrastructure of the precyst and cyst of Pneumocystis carinii was investigated by the freeze-fracture method. The most interesting point in this study is the alteration in the number of intramembranous particles (IMPs) of the plasma membrane during the formation of intracystic bodies. The IMP density of delimiting membrane originating from the precyst plasma membrane, which later becomes the plasma membrane of the intracystic body, was low. In contrast, that of the plasma membrane of the intracystic body was as high as that of a typical trophozoite. These results suggest that the protein content of the plasma membrane increases rapidly when the delimiting membranes fuse and intracystic bodies are organized. The plasma membrane of the ruptured cyst, on the other hand, showed lower density and heterogeneous distribution of IMPs. Since all other stages of P. carinii showed homogeneous distribution of IMPs, the denaturation of the plasma membrane after excystation of intracystic bodies is indicated.