Stretching of skinned fibres of frog skeletal muscle beyond the overlap of the thin and thick filaments followed by release to resting length results in disorganization of the thin filaments at the A-I junction of a sarcomere (Higuchi et al. (1988) J. Muscl. Res. Cell. Motility 9, 491-8). Immunoelectron microscopic observations showed that the binding sites of antibodies against connectin (titin) returned to the original position after extreme stretch and release but those of anti-nebulin antibodies were largely disorganized. The binding sites of anti-connectin antibodies moved within an I band with the change in sarcomere length, but those of anti-nebulin antibodies did not. Nebulin remained in the I band at extreme stretch. Thus connectin filaments appear to be responsible for maintaining mechanical continuity of a sarcomere and appear to behave independently of thin filaments. It is suggested that nebulin is localized in the I band but not in the A band and is associated with thin filaments but not with the elastic structure of myofibrils.