The proposed life cycle of the opportunistic organism Pneumocystis carinii (PC) includes the formation of a thick-walled cyst containing haploid progeny. The progeny cells mature within the cyst and are somehow released, leaving a collapsed empty cyst behind. Pneumocystis carinii cysts containing 8 intracystic bodies (ICB) as well as empty collapsed cysts have been commonly observed. In this study a single pore in the cyst walls of PC was observed by transmission electron microscopy in infected rat-lung preparations. Presence of a single pore in the cyst wall of an empty but noncollapsed cyst and in collapsed, empty cysts suggests that pore formation may be a mode of excystation for ICB of P. carinii.