A manually installable polymeric plug useful for hydrostatic pressure testing of polymeric pipe or tubing having a flexible wall, the plug having a substantially rigid polymeric handle with a threaded cylindrical section that is engageable with the polymeric pipe or tubing, and an elastomeric sealing member that extends forwardly of the first element and is insertable into the open end of the polymeric pipe or tubing and blocks fluid flow through the interior of the threaded cylindrical section, the elastomeric sealing member having a plurality of axially spaced sealing surfaces that provide a fluid seal between the sealing member and the inside wall of the polymeric pipe or tubing.