Studies of density body size relationships report generalities in the structure of communities and suggest patterns of energy use among species of different sizes. Most density body size relationships, however, have been developed from data on populations throughout the world and are expected, therefore, to provide only an approximate description of any specific community. In this study, we test 1) whether log-linear density body size relationships can also be used to describe local communities and 2) whether these relationships vary systematically with environmental conditions. Our analysis is based on population densities of phytoplankton, zooplankton, zoobenthos and fish species measured in 18 lakes worldwide. Mean annual population density (D, individuals m-2) decreases log-linearly with increasing species body size (M, μg fresh mass; log D = a + b x log M), with slopes and intercepts which vary among lakes (b = -1.10 to -0.74, a = 4.5 to 6.9). In contrast with studies which focus on only one group of organisms (e.g. birds, insects, mammals), we found that local communities are well described by density body size relationships (r2 = 0.78-0.98) when more complete communities are considered. The slopes of density body size relationships measure the relative density of small to large species, and tend to be shallower (lower relative density of small species) in lakes near cities, but more steeply negative (greater relative density of small species) in deep, highly productive lakes which flush rapidly. The intercepts measure average population density and are positively related to primary productivity. Such differences in size structure among aquatic communities are large enough to suggest shifts, for example, in the relative energy use of small and large species.