The topography and organization of cells lining the peritoneal surface of the diaphragm in normal and intraperitoneally injected mice have been studied by transmission and scanning electron microscopy and freeze-fracture replication techniques. The mesothelial cells of the peritoneal surface of the diaphragm are organized into two discrete populations. A population of cuboidal cells are found in areas over the lymphatic lacuna (roof covering submesothelial lymphatic vessels) while the remaining areas (not covering lymphatic lacunae) of the diaphragm are lined by extremely flattened cells. The apical surfaces of both cuboidal and flattened mesothelial cells are covered with numerous microvilli. The lateral borders of the cuboidal cells are characterized by the presence of numerous filamentous processes which interlace with similar structures from adjacent cells to form intercellular gaps which overlie a thin layer of connective tissue. In addition to the intercellular gaps, special circular pores are found between the margins of several neighboring cells. The observations suggest that these circular pores are formed when cell margins of several lymphatic endothelial cells span the submesothelial connective tissue to form intercellular junctions with cell margins of several neighboring mesothelial cells lining the peritoneal surface of the diaphragm. The intimate intercellular contact between these two different cell types provides an open channel between the peritoneal cavity and the lumen of the diaphragmatic lymphatic vessels (lymphatic lacunae). It is suggested that these circular pores correspond to the “stomata” of von Reckling-hausen. Intraperitoneally injected colloidal particles (biological carbon) and red blood cells are rapidly removed from the peritoneal cavity via the circular pores (stomata).