A new instrument capable of recording the load-extension behavior of a yarn in cyclic testing is described. The instrument is constructed in such a fashion that it is possible to vary both the humidity and the temperature of the air in which the yarn is tested. Frequency and amplitude are also variable over a limited range. It is shown that the range of fatigue lifetimes of yarns in cycling conforms to the Weibull distribution, which is based on the theory of extreme values. Experimental evidence indicates that increased "plasticization" brought about by heat and/or water tends to increase the fatigue life of fibers. The failure of twisted yarns in fatigue testing is different than that observed in un twisted structures This fact points out the difficulty in extrapolating the results pre sented here to actual tire cord failures.