The proximal accessory flexor (PAF) of the myochordotonal organ (MCO) in the meropodite of crayfish walking legs contains two populations of muscle fibers which are distinguishable by their diameters. The large accessory (LA) fibers are 40-80 micrometer in diam and are similar in ultrastructure to other slow crustacean fibers. The small accessory (SA) fibers are 1-12 micrometer in diam and have a unique myofilament distribution at normal body lengths. There is extensive double overlap of thin filaments at these lengths, and some of them form bundles that may extend the length of the sarcomere. In the middle of the sarcomeres, thick and thin filaments are totally segregated from each other. When the fibers are stretched to lengths beyond double overlap length, the myofilament patterns are conventional. The segregated pattern is reestablished when stretched fibers are allowed to shorten passively. The length-tension relationship of the SA fibers is described by a linear ascending branch, a plateau, and a linear descending branch. The ascending branch encompasses normal body lengths from slack length (Ls) with maximum double overlap to the length at which double overlap ceases (1.8 X Ls). The descending phase is comparable to that of other skeletal muscles. That is, tension decreases in proportion with the reduction in thick-thin filament interdigitation (2 X Ls to 3 X Ls).