The transport of a flexible thread is effected by means of an injector for inserting a weft thread into a weaving mechine. In order to therewith keep the air consumption as low as possible the diameter of the discharge tube is made as small as possible. For nevertheless increasing the force imparted to the thread and thereby its directional stability the invention uses with an injector for subsonic inlet flow of the gas a discharge tube which widens along a very small conical angle, such that over its full length the gas obtains the speed of sound. With an injector for supersonic flow of the inlet gas this supersonic velocity is maintained along the full length of the discharge tube likewise by widening the discharge tube along a very small conical angle.The directional stability of the thread is further increased if, by keeping constant the cross-sectional area of the discharge tube, it is flattened in the direction perpendicular to the plane of the warp threads.