Fish meal may contain unknown growth factors that have yet to be identified for their physiological role. Carnosine is a histidine-²-alanine dipeptide found in muscle and nervous system tissue which has been demonstrated to have biological activity, but its physiological role is not well defined. A 9-week feeding study was conducted comparing diet FM, a 100% fish meal protein control diet, to fish fed three plant protein diets: diet SPI, 100% of the fish meal replaced with soy protein isolate; diet SPI + AA, diet SPI supplemented with methionine, lysine, threonine and glycine to diet FM levels; and diet CSN, diet SPI + AA supplemented with carnosine. Feeding diet SPI resulted in significant differences in feed conversion ratios (FCR), percent gain and protein retention efficiencies relative to fish fed diet FM. Feeding diets SPI + AA and CSN resulted in FCRs, percent gains and protein retention efficiencies that were not significantly different from fish fed diet FM. Fish fed diets SPI, SPI + AA and CSN resulted in reduced muscle ratio (MR) and feeding diets SPI + AA and CSN resulted in increased intraperitoneal fat ratio (IPFR) relative to fish fed diet FM. Supplementing carnosine to an all-plant protein diet resulted in elevated plasma carnosine and increased muscle free pool anserine. Feeding diets SPI, SPI + AA and CSN resulted in reduced muscle development and increased calpain induced proteolysis. In conclusion, carnosine supplementation did not significantly improve the 100% plant protein diets in regard to the measured growth characteristics above the amino acid supplemented treatments and other unidentified factors may be limiting in the diet causing the reductions in MR and elevated IPFR. Highlights º Carnosine supplementation does not improve growth performance of trout fed an all-plant-protein diet. º Cellular muscle development was not enhanced by carnosine supplementation. º Feeding trout an all-plant-protein diet results in increased muscle protein degradation via calpain induced proteolysis.