In the I-band of skeletal muscle sarcomeres, the elastic region of titin consists of immunoglobulin (Ig) domains, and non-modular regions rich in proline, hydrophobic, and charged residues (PEVK). Using immunoelectron microscopy with sequence-assigned monoclonal antibodies, we demonstrate that extension of the Ig regions in M. psoas occurs largely at sarcomere lengths between 2 and 2.8 micron, decreasing in slope towards higher lengths. The Ig domains do not unfold. Above 2.6 micron, length changes are increasingly due to the PEVK-rich regions. We therefore propose that rubber-like properties of the PEVK-rich regions are mainly contributing to skeletal titin elasticity.