Technical Abstract: Rainbow trout were reared on a diet formulated to supply the required protein from either fishmeal or protein meal sources. From liver and muscle tissue samples RNA and protein were isolated and analyzed for the expression of a number of muscle regulatory and protein degradation genes and enzymatic activity for proteins involved in degradation pathways for protein proteolysis. Only MyoD2 showed significant differences in expression between the two diets and no significant increases or decreases over the course of the experiment were determined for MyoD2 or the other muscle factors. For the degradation genes significant changes in expression were determined for calpain1 and calpastatinL. CalpastatinL also showed a significant increase in expression over the course of the experiment in the muscle of fish fed a fishmeal diet and significant decrease in expression in the liver of fish fed the fishmeal based diet. Differences in proteasome enzyme activity were found between diets in the liver and muscle of fish and for caspase-3 activity in muscle. These findings suggest that diets replacing fishmeal with plant material can have some effects on protein turnover in muscle and that some degradation pathways are differentially regulated during the growth of rainbow trout.