Colored tracers, injected intraperitoneally in mice, are taken up by diaphragmatic lymphatics, outlining their large, terminal cisterns, the so-called lacunae. The lacunae occur exclusively on the muscular portion of the diaphragm. The mesothelium covering non-lacunar and lacunar areas of the muscular portion was examined with the SEM. Mesothelial cells overlying non-lacunar areas are extremely flat, and their boundaries are indistinct. Mesothelial cells overlying lacunae protrude towards the lumen of the peritoneal cavity and have distinct outlines. There are openings or stomata, 4-12 μm in diameter, between them. Some of the stomata overlie a deep pit; others overlie a shallower pit in which the surface of another cell can be seen beneath the opening. It seems likely that the bulk of the fluid draining from the peritoneal cavity passes through these stomata into underlying lymphatic lacunae.