1. Relations between sarcomere length, tension and time course of isometric twitches at 20 degrees C were determined for thirty-two sartorius muscles from Rana temporaria.2. The maximum isometric twitch tension per unit cross-sectional area of muscle ranged from 0.56 to 2.2 kg/cm(2) at 20 degrees C and initial sarcomere length about 2.1 mu. This variation was not correlated with the corresponding measure of tetanic tension.3. The maximum isometric twitch tension per unit cross-sectional area of muscle at 2.1 mu sarcomere length was directly correlated with twitch contraction time and inversely correlated with the cross-sectional area of the muscle.4. The isometric twitch contraction was potentiated with increase in initial sarcomere length from about 2.1 to about 2.8 mu at 20 degrees C, and the degree of potentiation was inversely correlated with the maximum isometric twitch tension per unit cross-sectional area of muscle at 2.1 mu sarcomere length.5. The sarcomere length: twitch tension relation is labile and may be altered by changes in temperature and the after-effects of repetitive stimulation.6. Variation in twitch contractions at different sarcomere lengths are discussed in connexion with excitation-contraction coupling and the degree of activation of muscle fibres during twitch contractions at 20 degrees C.