The relation existing between the twist and resistance to repeated extensions of man-made-fibre rotor-spun yarns is studied. Experiments on acrylic-fibre, polyester-fibre, and viscose staple-fibre rotor-spun yarns are reported, and it is shown that, for these fibres, the maximum resistance to repeated extension is consistent with generally low twist multipliers, although the level of the resistance is dependent on both the fibre type and the linear density of the yarns. Some complementary tests concerning the relation existing between twist and tensile strength for cotton, polyester-fibre, and acrylic-fibre rotor-spun yarns are also reported. It is shown that, whereas for cotton yarns the twist for maximum strength is found for a high twist multiplier, for man-made-fibre yarns the optimum twist occurs at low values of the twist multiplier.