MTM1, MTMR2, and SBF2 belong to a family of proteins called the myotubularins. X-linked myotubular myopathy, a severe congenital disorder characterized by hypotonia and generalized muscle weakness in newborn males, is caused by mutations in MTM1 . Charcot-Marie-Tooth types 4B1 and 4B2 are severe demyelinating neuropathies caused by mutations in MTMR2 and SBF2/MTMR13 , respectively. Although several myotubularins are known to regulate phosphoinositide-phosphate levels in cells, little is known about the actual cellular process that is defective in patients with these diseases. Mutations in worm MTM-6 and MTM-9, myotubularins belonging to two subgroups, disorganize phosphoinositide 3-phosphate localization and block endocytosis in the coelomocytes of Caenorhabditis elegans. We demonstrate that MTM-6 and MTM-9 function as part of a complex to regulate an endocytic pathway that involves the Arf6 GTPase, and we define protein domains required for MTM-6 activity.