The vertical distribution patterns of the dominant zooplankton in the vicinity of Marguerite Bay on the west side of the Antarctic Peninsula were studied during austral fall of 2001 and 2002, using net and concurrent environmental data. Vertical distributions of zooplankton usually were similar to those reported for other Antarctic regions. Maximum abundances of the copepods Ctenocalanus spp. and Calanus propinquus, the euphausiids Euphausia superba, Euphausia crystallorophias, and Thysanoessa macrura, and appendicularians primarily occurred in shallow Antarctic Surface Water (<100 >m) or the upper pycnocline. The copepod, Oncaea spp., mysids, and ostracods had the deepest distributions (>250 m), in warmer modified Circumpolar Deep Water. Other dominant copepods (Calanoides acutus, Metridia gerlachei, Oithona spp., Paraeuchaeta spp., and Rhincalanus gigas), pteropods, and chaetognaths had depths of maximum abundance within the pycnocline or in deeper warmer waters. Overlapping depth distributions suggest that E. superba would have the highest prey encounter rates with M. gerlachei, Ctenocalanus spp., C. propinquus, and Oithona spp. during fall, although most of the copepod community was deeper than the euphausiid community. Even though the three euphausiid species occupied similar depth ranges on average, at any given location E. superba, E. crystallorophias, and T. macrura depths of maximum abundance often did not overlap, suggesting vertical habitat partitioning behavior. The vertical patterns of copepods, euphausiids, amphipods, and mysids did not have a consistent association with the distributions of pigments, temperature, salinity, or density. Instead, the observed vertical distributions are mainly attributed to different behaviors, including seasonal vertical migration to deeper water for overwintering (i.e., C. acutus, R. gigas, ostracods, chaetognaths, pteropods) and vertical habitat partitioning to reduce competition (i.e., euphausiids). Migration into deep water and aggregation behavior (i.e., euphausiids) also reduce the risk of predation.