In order to best predict the consequences of interaction between domestic and wild fish populations, the physiological and genetic differences that have occurred during the domestication process should be understood. Through production of multiple generations (F 1 and F 2 backcrosses) of domestic and wild rainbow trout (Oncorhynchus mykiss) hybrids, the present study examines genetic changes that have occurred during domestication, as well as the action of these changes when introgressed back into a wild genome. A strong correlation observed between the proportion of domestic genes within the genotype and growth indicates that additive gene action is responsible for a large amount of the phenotypic variability.