Technical Abstract: Feed costs represent nearly half the total cost of production for most animals, including fish, so improving growth and feed efficiency are top priorities for improving profitability of fish culture. Improved feed efficiency means better utilization of feed for production of meat, in other words increasing the pounds of growth per pound of feed consumed. Individual fish are being monitored for the proportion of feed energy that is used for growth and the proportion used for metabolic maintenance. There were significant differences in metabolic costs between different strains of rainbow trout. Efficiency can be improved by minimizing the amount of energy going into metabolic maintenance. In this article, work begun to identify fish that grow more efficiently is described.