It is shown that the stray loss in induction motors, at fractional slips greater than about 0.25, is almost entirely due to tooth ripple in the zigzag leakage flux. The stray loss is large; it can double the torque at a slip of 2.0 and can lead to serious errors when calculating accelerating and braking performance. In the normal operating range, i.e. slip less than 0.05, although the stray loss is much lower it is still appreciable, and the component associated with tooth ripple in the main flux is of importance. Experimental evidence, both from actual machines and from high-frequency loss in samples of laminations, indicates that the zigzag leakage flux component of the loss is approximately proportional to (speed)1.6and (current)1.9, and the results of the American Standard reverse-rotation test extrapolated according to this law are sufficient, together with the no-load stray loss, to predict the total stray loss over the whole practicable speed range.A simple method for calculating the stray loss is given and its limitations are indicated. Calculated values of the loss are compared with the measured losses, but no definite conclusions as to the accuracy of the calculations can be drawn until more is known of the highfrequency loss in machine laminations.