P. carinii is a protozoan which induces an often fatal pneumonitis in a variety of compromised patients. The ultrastructure of P. carinii was studied in a male infant with pneumocystitis pneumonia associated with hypogammaglobulinemia. Four principal structural varieties-small trophozoites, large trophozoites, mature cyst and empty cyst were identified. The ultrastructure of these organisms was similar to the cases previously reported. Relevance of the morphologic findings to the functional aspect were discussed.