The dominant invertebrate zooplanktivore in Lake Baikal, Russia, is the pelagic amphipod Macrohectopus branickii. We followed the dynamics of an aggregation of this amphipod in Barguzin Bay, middle Lake Baikal, between 27 and 30 September 1989, using a 200 kHz echosoun-der and vertical net tows. Correlations between amphipod biomass and volume back-scattering yielded a target strength of −66.8 dB/g or −82 dB/individual (15 mm, 30 mg animal). This is similar to results from theoretical scattering models. Macrohectopus were aggregated in a 29 km 2 large patch over bottom depths of 150 to 200 m (density 73 g/m 2) during the day. This patch spread out to 40 km2 during the night (density 64 g/m 2). Density estimates for the whole bay were 9.1 g/m 2 (night) and 8.9 g/m 2 (day). Total area surveyed was 415 km 2. The amphipod migrated from daytime depths of 100–200 m to nighttime depths of 20–70 m. Both the evening ascent and the morning descent lasted about 1.5 h, corresponding to a migration velocity of 1 m/min. Larger females were found deeper than smaller females both day and night. Reaction of Macrohectopus to a flood light suggested that the animals avoided light levels brighter than 0.0001 lux. The prey of Macrohectopus (smaller zooplankton) were primarily distributed above 50 m depth both day and night. These data indicate remarkable similarities with the migration patterns of mysids, the ecological analog to Macrohectopus in many large northern lakes. This is the first study to continuously follow the diel dynamics of the amphipod and to map the size of an amphipod aggregation using hydroacoustics.