Technical Abstract: Both environmental and economic concerns have led to the development of fishmeal free diets for salmonids. The ability of rainbow trout to efficiently utilize plant-based diets for growth, and the genetic variation within populations for this trait have not been thoroughly examined. In a previous study we used microsatellites to determine the pedigree of the top 1% and bottom 1% of progeny in a commercial growth trial of 20 full-sib families reared in a common environment. Half of the fish from each family was fed a standard fish-meal based diet and the other half was fed a plant-protein (gluten) based diet. The family rankings were similar when either diet was fed, indicating that no genotype x diet interaction was present in that commercial trout strain. In the present study, growth of a pedigreed population from another commercial strain was assessed while feeding both plant-based and traditional fish-meal diets. Also, in this study soybean oil was included in the plant protein meal diet. Families (95 full-sib or 47 half-sib) were reared in a common environment and parentage assignment performed on approximately 1000 fish fed each diet. The fish fed control (fish meal/oil) diet were significantly larger than the fish fed the plant diet. Unlike the previous study, a significant genotype x diet effect was detected which accounted for 5% of the random variation and the genetic correlation for growth on the two diets was 73%. Differences between the two studies may have been caused by the addition of soybean meal and oil to the plant-based diet used and the experimental design that enabled quantitative assessment in the current study. The use of a different strain may have contributed to the difference as well. The genetic variation we identified can be explored to identify and select for genes involved in improved utilization of plant-based diets containing soybean meal and oil.