Summary—The ultrastructural morphology ofPneumocystis cariniiobtained from nonimmunosuppressed rabbit is described in details. Golgi complex and primary lysosomes ofP cariniiare described here for the first time. They are easily revealed by the zinc iodide‐osmium tetroxide cytochemical reagent. Thiamine pyrophosphatase and β‐glycerophosphatase activities are found in the parasite but cytidine 5′ monophosphatase activity is not observed. A weak thiamine pyrophosphatase activity is detected in Golgi vesicles. An endomembranous saccular structure, present from the intracystic body stage to the precystic stage, apparently plays the role of secondary lysosome. A second type of endomembranous saccular structure, only present in the well developed trophozoitic and precystic forms is also described. The presence of carbohydrates in the cell wall of the parasite was demonstrated by periodic acid‐thiosemicarbazide‐silver proteinate staining and lectin concanavalin A labeling. The development of Golgi vesicles preceded the transition from double‐layered to three‐layered