Elastic proteins in the muscles of a nematode (Caenorhabditis elegans), three insects (Drosophila melanogaster, Anopheles gambiae, Bombyx mori) and a crustacean (Procambus clarkii) were compared. The sequences of thick filament proteins, twitchin in the worm and projectin in the insects, have repeating modules with fibronectin-like (Fn) and immunoglobulin-like (Ig) domains conserved between species. Projectin has additional tandem Igs and an elastic PEVK domain near the N-terminus. All the species have a second elastic protein we have called SLS protein after the Drosophila gene, sallimus. SLS protein is in the I-band. The N-terminal region has the sequence of kettin which is a spliced product of the gene composed of Ig-linker modules binding to actin. Downstream of kettin, SLS protein has two PEVK domains, unique sequence, tandem Igs, and Fn domains at the end. PEVK domains have repeating sequences: some are long and highly conserved and would have varying elasticity appropriate to different muscles. Insect indirect flight muscle (IFM) has short I-bands and electron micrographs of Lethocerus IFM show fine filaments branching from the end of thick filaments to join thin filaments before they enter the Z-disc. Projectin and kettin are in this region and the contribution of these to the high passive stiffness of Drosophila IFM myofibrils was measured from the force response to length oscillations. Kettin is attached both to actin near the Z-disc and to the end of thick filaments, and extraction of actin or digestion of kettin leads to rapid decrease in stiffness; residual tension is attributable to projectin. The wormlike chain model for polymer elasticity fitted the force-extension curve of IFM myofibrils and the number of predicted Igs in the chain is consistent with the tandem Igs in Drosophila SLS protein. We conclude that passive tension is due to kettin and projectin, either separate or linked in series.