The Western Antarctic Peninsula (WAP) is a site of high krill abundance and a likely source region for krill populations found to the north and east of the area. The U.S. GLOBEC Southern Ocean program studied factors that contribute to the overwintering success of krill in the region of Marguerite Bay, WAP. A MOCNESS net system was used to sample the vertical distribution and abundance of zooplankton relative to physical features (hydrography and circulation) during four broad-scale survey cruises in the fall and winter of 2001 and 2002. Four species were found throughout the study area on all four cruises: Euphausia superba, Euphausia crystallorophias, Euphausia triacantha, and Thysanoessa macrura. The species had significantly different horizontal and vertical distributions. Both E. superba and T. macrura were broadly distributed throughout the area, but the central 50% of their vertical distributions were distinct with E. superba most abundant in the upper 100 m in the coldest, freshest water (average temperature and salinity: −1.13 °C; 33.80) and T. macrura occurring between 100 and 250 m (at 0.01 °C; 34.228). E. crystallorophias had a more coastal distribution and was usually found deeper in slightly warmer and saltier water (−0.44 °C; 33.9982) than E. superba and either overlapped or was above T. macrura in depth. E. triacantha was much rarer and sporadically distributed in the study area and was found substantially deeper (center of distribution about 300 m) in the warmest saltiest water (1.40 °C; 34.65) than the other three euphausiid species. Larval distributions for E. superba indicated that at least some proportion of the populations resulted from reproduction and development on the continental shelf, and not solely from offshore reproduction and transport onto the shelf. A neutral particle tracking model was used to gain insight into the relative importance of shelf versus off-shelf origins for the larvae. The results suggest that a combination of offshore and onshore reproduction can account for the observed E. superba larval distributions in the U.S. SO GLOBEC study site.