Two experiments were conducted to select rainbow trout families for improved feed utilization efficiency with soybean-based diets. In the first experiment, a total of 1200 fish (32.7±9.4 g , average±SD) belonging to 15 families (CX-118, CX-125, CX-134, CX-135, CX-137, CX-138, CX-141, CX, 143, CX, 144, CX-145, CX-146, CX-147, CX-148, CX-149, and CX-152) of rainbow trout (80 fish/family) were PIT tagged and randomly distributed in four tanks (1 m3 ). Fish were reared in this environment for four months and went through two periods of feed deprivation (FD) and two periods of refeeding (RF). During the RF periods fish were fed at satiation with an all-plant extruded diet . After each period, all fish in each tank were anesthetized, PIT tangs were scanned, and weight and lengths were measured . Fish performance was classed as FD-, FD+, RF- and RF+ for fish exhibiting loss (FD) and gain (RF) of weight relatively lower (-) and higher (+) than the population mean and separated into four triplicate groups FD-/RF-, FD+/RF+, FD-/RF+ and FD+/RF- . In the second experiment, another group of 1200 rainbow trout (32.0±6.7 g, average±SD ) belonging to the same 15 families were individually PIT tagged and distributed into two groups of 40 fish/tank. The two groups of fish were fed at satiation with two diets, Diet 1 was supplemented with lyophilized powder of spirulina whole cells and Diet 2 was supplemented with lyophilized powder of spirulina whole cells labeled with U-15 N. After 18 days of feeding, all fish were scanned, and weight and length were measured. Also, 0.4 mm diameter p unches were used to take muscle samples from each fish to analyze for 15N isotopes . Results for the first experiment showed family CX-138 had significantly lower weight loss during FD and family CX-125 had the highest weight loss during this period (P<0.05). For the RF period, family CX-148 had the highest WG among all the families. Families CX-125 and CX-146 were in second place with the highest WG compared to other groups and family CX-143 showed the lowest WG during RF . Results for the second experiment showed a similar trend for weight gain (WG %) as the CX-148 family had the highest WG but with no significant differences with the CX-125 group (P>0.05). The results for 15N isotope analysis in fish muscle will be presented at the conference. Overall, the results of the present study, so far, have demonstrated that there are substantial genetic-based variations among different families of rainbow trout in utilizing soybean-based diets.