Abstract: A fitting, for pipe of flexible plastic material, having improved sealing and mechanical holding characteristics. The fitting includes a main body having an external screw thread terminating at a shoulder from which a coaxial, reduced-diameter, nipple projects. Annular saw tooth or buttress ribs on the nipple are dimensioned to have a slip fit into the interior of a plastic pipe section. A collet is slideable on the exterior of the pipe section into abutment with the shoulder and is radially compressed by a collet sleeve threaded onto the external threads of the fitting body. Axial slots in the collet open alternately at opposite ends of the collet so that radial compression is achieved over the entire length of the collet. Saw toothed or buttress annual ribs on the collet interior bite into the exterior surface of the pipe section in opposition to the ribs on the inserted nipple, the pipe wall being thereby radially clamped between the collet and nipple.