Pneumocystis cariniiinfected rat lungs were postfixed with a mixture of OsO4and K4Fe(CN)6. A marked improvement in staining of cell membranes, endoplasmic reticulum, nuclear membranes and glycogen was observed. These improvements were seen in both the trophic and cystic forms of the organisms. The addition of K4Fe(CN)6did not improve the staining of cell walls, microtubules or ribosomes. Trophozoites were seen attached to both type 1 and type 2 pneumocytes by filopodia and/or intercalation of the cell body ofP. cariniiwith the host lung cells. It is expected that the improvement in ultrastructural detail will allow better understanding of the ultrastructure ofP. cariniiand provide insights into the modes of action of various antimicrobial compounds on this organism.