1. The stability of sarcomere lengths along single twitch fibres from frog muscles was examined during fixed-end tetani, using a spot follower apparatus to monitor the length of a central segment. 2. Internal movement, with most of the fibre lengthening and small regions at the ends shortening as the contraction proceeded, was always seen at fibre lengths beyond those corresponding to a sarcomere length of 2.3 micrometer. 3. The rate of lengthening of the central region was fastest during the slow phase of tension rise (creep) but continued at a slower rate throughout the tetanus. These observations are in accord with the idea that progressive development of sarcomere non-uniformity is responsible for the creep phase. 4. Observations at various muscle lengths of the rate of decay of tension and the duration of the slow phase of relaxation suggest that movement during relaxation is due to sarcomere length non-uniformities and variations of decay rate with sarcomere length. 5. The rate of tension fall after stimulation ceases in an isometric sarcomere, and the factors which determine that rate, are discussed in view of evidence from fixed-end and length-clamped tetani, and recently reported experiments using aequorin.