The population energetics of two species of amphipods was studied at several depths in Marion Lake, BC. Hyalella azteca was abundant at a depth of 1.0 m, (mean summer standing crop, 1952 animals/m2 but became rare (75 animals/m2 at depths greater than 2.5 m. Growth rates, res- piration rates, and hence energy flow were decreased by the lower ambient temperatures of deeper water, On an annual basis, a mean standing crop of 1.1 Kcal/m2 of Hyalella assimilated 18.1 Kcal/m2 respired 14.1 Kcal/m2 and used 4.0 Kcal/m2 in production of growth, molts and eggs, Approximately 3/4 of the annual energy flow was completed between June and October, The ecological efficiency of Hyalella populations was 10, the net production efficiency, 22%, and the net population growth efficiency, 12%-13%, The mean summer density of Crangonyx remained constant with depth (about 249 animals/m2). Annual energy flow and production were not appreciably affected by lower temperatures in deeper water. On an annual basis, a mean standing crop of 0.7 Kcal/m2 assimilated 7.7 Kcal/m2 respired 6.3 Kcal/m2 and used 1.4 Kcal/m2 in production Crangonyx energy flow was fairly constant throughout the year. The ecological efficiency of Crangonyx was 8%, the net production efficiency, 17%-19%, and the population growth efficiency, 14%-15%. Annually, Hyalella populations assimilated only twice as much and produced about 3 times as much energy as did Crangonyx populations, but during the summer, Hyalella energy flow was 4 times that of Crangonyx. A synthesis of existing data on poikilotherm energetics suggests that life history phenomena are important in determining the relationship between annual population respiration and production. Also, annual production and assimilation were found to be closely related to the mean standing crop of various organisms. / Science, Faculty of / Zoology, Department of / Graduate