1. A comparative study has been made of the structure of the posterior and anterior latissimus dorsi muscles (P.L.D. and A.L.D.) of the chicken. Some of the contractile properties of these muscles, particularly of the A.L.D. muscle, have also been examined.2. In confirmation of earlier work, it is found that structural differences between these two muscles mainly reside in the organization of the T-system and the sarcoplasmic reticulum.3. In the P.L.D. muscle the T-system and sarcoplasmic reticulum are tubular networks of regular pattern, with frequent regions of contact between them in the form of the familiar triads. As described earlier, two series of triads are present in each sarcomere, near the level of the A-I boundary.4. The presence in the A.L.D. muscle of both T-system and sarcoplasmic reticulum has been established here by the combined use of electron microscopic and histochemical techniques. The appearance of these structures is considerably less regular in this than in the P.L.D. muscle, and the area of contact between them relatively small: in terms of contact area per sarcomere, about 15% or less of that in the P.L.D. muscle. Regions of contact are mainly in the form of dyads, but a few triads are also present.5. Contractures of the A.L.D. muscle in high K fluids have been recorded and their modification due to variation of the external concentration of Ca, K and Mg examined. The K contracture consists of two phases: an initial transient and a later maintained phase. The response of these phases to variation in the external medium is markedly different.6. Application of Ca-free fluids causes contracture tension of the late phase to subside within a few minutes whilst that of the initial phase declines completely only after several hours. High Mg concentrations depress the later but not the early phase of contracture.7. Tentative interpretations of both the morphological and physiological differences described have been proposed.