The use of a high frequency echo sounder and a midwater trawl in Babine Lake, British Columbia, in 1967 revealed a precise diel vertical movement of underyearling sockeye salmon. From early July to September, from about 1.5 hr after sunrise to 1.5 hr before sunset, the young sockeye were in two distinct layers at about 20 and 35 m, each layer about 6 m thick. About 1.5 hr before sunset the two layers began to ascend. About 0.5 hr after sunset all fish were within 3 m of the surface, and during darkness they were dispersed throughout the top 5–15 m with most fish being below the thermocline. Soon after the first light of dawn the fish usually tended to move toward the lake surface and then descended rapidly to the daytime depths.By early October the pattern had changed markedly. During daylight the fish were still found in roughly two layers at about 24 and 40 m. However, the evening ascent did not commence until about 0.5 hr before sunset, the ascent was much slower, and the fish did not come to the surface but were dispersed between 9 and 27 m. This change in behaviour was temporally associated with a cooling of the epilimnion and a decrease in intensity of feeding.Although the zooplankton of Babine Lake is composed of eight major species, only Bosmina coregoni and the calanoid copepod Heterocope septentrionalis displayed pronounced diel vertical movements. The former inhabited the surface waters during the day and were found in maximum abundance at 12–21 m at night. The latter was found at 21–30 m during the day and the surface waters at night. The major part of the zooplankton standing stock was found above 9 m.Young sockeye fed most intensively in the evening as they approached the surface and again at dawn just as they commenced the descent. At those times the most common food item was Daphnia longispina, followed by B. coregoni and H. septentrionalis. These three species were strongly selected by young sockeye, since they were numerically much less abundant than other limnetic zooplankton species. In August, terrestrial insects were occasionally of major importance. At midday the upper layer of sockeye (about 20 m) was feeding on H. septentrionalis whereas the lower layer (about 35 m) was not feeding or was feeding at a low intensity.The double layer of young sockeye during daylight occurred at least in the North Arm of Babine Lake and may be related to feeding. In July and August the fish in the upper layer had less food in the pyloric section of the stomach from 0900 to 1300 hr than did fish in the lower layer, whereas the volumes of food were similar in the two layers by late afternoon. Fish in the upper layer had much more fresh H. septentrionalis in the cardiac section from 1200 to 1800 hr than did fish in the lower layer. The ecological significance of the diel vertical movement is discussed briefly.