Zooplankton distributions during coastal upwelling in western Lake Superior were mapped with a high-frequency (192-kHz) sonar system that consists of an echosounder and a navigation receiver connected to a microcomputer. An acoustic transect after a September storm revealed a sloping sound-scattering layer, traceable more than 5 km offshore, in the thermocline between an upwelling water mass and a returning front of surface water. Interfacial shear produced short internal waves on the scattering layer, and regularly spaced plumes of high backscattering in the surface layer were attributed to stream lines of eddies that were transporting sound scatterers from aggregations in the thermocline to the lake surface. The sloping sound-scattering layer was due to high concentrations (up to 105 ind. m-3) of copepods, chiefly Leptodiaptomus sicilis. Backscattered sound (volume-scattering strength) was highly correlated with concentrations of microcrustacea in samples collected with nets, consistent with Rayleigh scattering theory for ideal elastic spheres the size of adult L. sicilis (target strength = -123 dB). Acoustic sampling revealed a skewed frequency distribution; low animal concentrations (< 1,000 m-3) occupied most (65%) of the zooplankton space, but a small proportion (< 10%) of the space contained very dense aggregations (> 105 m-3) with concentrations several orders of magnitude larger than the median. Infrequent patches of very high densities are probably a general feature of zooplankton populations, because the patchiness inferred from sound backscattering is similar statistically to distribution patterns in lakes and oceans sampled with conventional methods. The densest aggregations are likely to be the most important for foraging fishes but are so infrequent they are unlikely to be detected with conventional sampling methods.