Thermoplastic yarn is texturized by the known method of forwarding it in a heated condition to a crimping zone at the entrance of a stuffer chamber (7) so as to form a plug (8) of crimped yarn within the chamber and withdrawing the yarn from the other end of the plug at a speed which is related to the input speed. The improvement comprises deriving signals from the speed of the yarn plug (8) in the stuffer chamber (7) by means of a sensing wheel (11) which drives a gapped monitoring disc (13). The disc (13) operates to intercept a beam of light from a source (14) which is directed on to a photo-sensor (15). The output of the photo-sensor is supplied to an electrical controller (36) which measures the time during which light is received for each gap in the disc (13) to provide a measure of the speed of the yarn plug. The resultant signals are used to control the temperature of the yarn passing to the crimping zone in such a way as to maintain the speed and hence the quality of the bulk yarn substantially constant. For this purpose, the signals from the controller (36) are fed to a heater controller (28) to adjust the current to a heating element (5) in a chamber (4) through which steam passes before feeding the yarn (1) through a jet passage (2) leading to the chamber (7).