Titin is a >3000-kDa large filamentous protein of vertebrate-striated muscle, and single titin molecules extend from the Z disc to the M line. In its I-band section, titin behaves extensible and is responsible for myofibrillar passive tension during stretch. However; details of the molecular basis of titin's elasticity are not known. We have compared the motif sequences of titin elastic elements from different vertebrate species and from different regions of the molecule. The I-band titin Ig repeats that are expressed in the stiff cardiac muscle and those that are tissue-specifically expressed in more elastic skeletal muscles represent distinct subgroups. Within the tissue-specifically expressed Ig repeats, a super-repeat structure is found. For the PEVK titin sequences, we surveyed interspecies conservation by hybridization experiments. The sequences of the titin gene which code for the C-terminal region of the PEVK domain are conserved in the genomes of a larger variety of vertebrate, whereas the N-terminal PEVK sequences are more divergent. Future comparisons of titin gene sequences from different vertebrates may improve our understanding of how titin contributes to species diversity of myofibrillar elasticity. Within one species, different classes of Ig repeat families may contribute to elastic diversity of the titin spring in different segments.