IN the 35 years since A. C. Redfield’s classic paper(1), the use of elemental ratios has become widespread in marine and freshwater phytoplankton studies(2,3). But nutrient ratios have only recently been studied elsewhere in pelagic ecosystems, such as the producer-consumer interface(4,5). Here we report the results of the first study, to our knowledge, of N:P ratios in pelagic producers and consumers (phytoplankton and zooplankton) in lacustrine and marine habitats. The N:P ratio of phytoplankton was higher in lakes than in marine sites; however, N:P ratios were higher in marine zooplankton than in freshwater zooplankton. The elemental imbalance of the phytoplankton-zooplankton interaction (N:P-food-N:P-consumers) in lakes was positive and exceeded the negative imbalance in marine sites; thus P-deficient food may limit zooplankton growth in lakes but not in oceans. Stoichiometric calculations(6) indicated that consumer-driven nutrient recycling ratios in lakes may be 4-6 times higher than in marine systems. Consistent with this difference, phytoplankton P-limitation was more prevalent in lakes than in marine sites. Thus, the ecological stoichiometry of the zooplankton-phytoplankton interaction differs qualitatively in freshwater and marine ecosystems.