The role of stoichiometric food quality in influencing genotype coexistence and competitive interactions between clones of the freshwater microcrustacean, Daphnia pulex, was examined in controlled laboratory microcosm experiments. Two genetically distinct clones of D. pulex, which show variation in their ribosomal rDNA structure, as well as differences in a number of previously characterized growth-rate-related features (i.e., life-history features), were allowed to compete in two different arenas: (1) batch cultures differing in algal food quality (i.e., high vs. low carbon:phosphorus (C:P ratio) in the green alga, Scenedesmus acutus); (2) continuous flow microcosms receiving different light levels (i.e., photosynthetically active radiation) that affected algal C:P ratios. In experiment 1, a clear genotype x environment interaction was determined with clone 1 out-competing clone 2 under high nutrient (i.e., low food C:P) conditions, while the exact opposite pattern was observed under low nutrient (i.e., high C:P) conditions. In experiment 2, clone 1 dominated over clone 2 under high light (higher C:P) conditions, but clonal coexistence was observed under low light (low C:P) conditions. These results indicate that food (nutrient) quality effects (hitherto an often overlooked factor) may play a role in microevolutionary (genotypic) responses to changing stoichiometric conditions in natural populations.