By using an elemental-stoichiometry approach to zooplankton-phytoplankton interactions, we compare elemental composition and aspects of nutrient deficiency across a variety of marine and freshwater ecosystems. During 1992 and 1993 we sampled a total of 31 lakes (in northern Wisconsin and Michigan and the Experimental Lakes Area of northern Ontario) and 21 marine stations (at seven estuarine, coastal, and open-ocean sites in the Atlantic and Pacific) for elemental composition of zooplankton, seston, and dissolved components. Relative degree of nutrient deficiency was assessed by phytoplankton dark uptake of ammonia and phosphate, as well as growth response of phytoplankton to N and P addition. Marine and freshwater systems differed greatly in N and P concentrations, N:P stoichiometry, and the distribution of N and P within dissolved, seston, and zooplankton pools. Particularly notable was the high proportion of N and, especially, P that was incorporated in the particulate fraction (seston + zooplankton) of lakes compared to marine sites. In freshwater systems, Daphnia spp., which have low body N: P, dominated zooplankton communities when seston C:P and N:P were also low, and calanoids that tend to have high body N:P dominated when seston C: P and N: P was high. This relationship between zooplankton community composition and seston elemental stoichiometry supports arguments for the importance of food quality constraints on zooplankton growth in freshwater systems. Such patterns were not seen in marine systems.