Rhynchospora rubra was found to have a low CO2 compensation point, high δ13C value, Kranz leaf anatomy, starch present in the bundle sheath cells and narrow interveinal distance. These observations suggest thatR. rubra is a C4 plant. A further anatomical survey revealed seven otherRhynchospora species presumably having the C4 photosynthetic pathway. In the family Cypraceae C4 plants therefore occur in the tribe Rhynchosporeae as well as in the Scirpeae and Cypereae. The C4 species ofRhynchospora have a normal Kranz type of leaf anatomy, although the C4 species ofCyperus andFimbristylis presently known have an abnormal one in which the mestome sheath without chloroplasts is interposed between the Kranz tissue and the rest of the chlorenchyma. Thus inRhynchospora the Kranz tissue is in direct contact with the rest of the chlorenchyma, and it is suggested that the Kranz tissue may be homologous with the mestome sheath.