Ultrastructure of the renette cell and caudal glands was studied in the free-living aquatic nematode Sphaerolaimus gracilis. The renette cell occurred posterior to the esophageal-intestinal junction and opened through an ampulla to a ventral pore behind the nerve ring. The caudal gland system of the tail consisted of two gland cells opening through separate pores and 2 to 3 other gland cells of a different type opening through a common pore. The renette cell and the two caudal gland cells were similar and both contained secretory granules, 0.5-1.5 mum in diameter. The material released attached the nematode to the substrate. The renette ampulla was surrounded by a specialized cell, the ampulla cell, which had characteristics of myoepithelium. A plug or valve structure connected to the ampulla cell may regulate the output of the secretory material. The ampulla cell is able to contract and thus is probably under direct neuronal control. Other cells in the renette ampulla region of body cavity were termed supporting cells. Living, cold-relaxed nematodes were attached to sediment particles in the renette pore region and at the tail tip. Release from sediment particles was mechanical at the renette cell discharge site but appeared to be chemical at the caudal gland. In behavioral experiments, nematodes in a water current had the ability to release a thread from the caudal glands while maintaining contact with a sediment particle attached to the tail end. If the thread was strong enough, it also could be used to change location. Nematodes anchored by the thread from the caudal glands to a sediment particle could float in water currents until they attached themselves to another sediment particle with the help of secretions from the renette cells.