Post-mortem softening of fish tissue often results in low yield and decreased product quality. In this study, proteolytic profiles of trout stored 5 days on ice were obtained by SDS–PAGE. The link between protein band intensities and firmness of trout fillets was examined through a correlation study. In parallel, trout extracts were incubated with cathepsin B, cathepsin L and cathepsin D, alone or in combination, in order to evaluate the effect of each cathepsin on the texture-related proteins. Proteins from both myofibrillar (α-actinin, actin, MLC1, MLC2, and N-terminal 70 kDa MHC fragment) and sarcoplasmic (glycogen phosphorylase, creatine kinase, and TPI) fractions correlated closely with firmness. Cathepsins D, B and L affected, respectively, 10, 9 and 4 out of the 17 protein bands correlating with firmness, and most changes induced by cathepsin D were unfavourable to firmness. This implies that cathepsin D is likely to be involved in textural change of trout, due to breakdown of the muscle structure.