Abstract: A method and device for forming grooves in a thermoplastic pipe, by surrounding said pipe with a mould having one or more grooves in its interior wall and positioning one or two annular chambers into said pipe, a flexible sealing circular body being sealingly movable in said chamber, and the outer circumference of said body in its free state being substantially equal to the inner circumference of the mould part facing said ring minus about twice the wall thickness of the pipe in its final state. The circular body can be retracted into the annular chamber which is provided with a bottom part having a clover-leaf-like cross section, the circumference of said cross section being substantially equal to the inner circumference of said circular body.