Recent laboratory work on food quality constraints on zooplankton growth and reproduction, as well as several examples of weak effects of food-web manipulations on lower trophic levels in lakes with phosphorus-deficient phytoplankton, suggests that food quality effects may have currently unappreciated effects on zooplankton success and food-web interactions under field conditions. We experimentally manipulated two factors that we anticipated might play a role in suppressing Daphnia in P-limited lakes - the quality of phytoplankton food and the presence of the invertebrate predator Chaoborus punctipennis. We used a two-factor design, manipulating food source and presence of Chaoborus, and measured growth rate, survivorship, and fecundity of Daphnia rosea neonates incubated at fixed food levels in flow-through growth chambers. D. rosea grew significantly faster and was significantly more fecund when fed seston from a high-food quality lake (Lake 979) relative to a treatment fed seston from a low-food quality lake (Lake 110). Chaoborus reduced survivorship of D. rosea but the food source-predator interaction term was not significant, indicating that invertebrate predation and phytoplankton food quality did not influence Daphnia populations synergistically in this experiment. A second experiment was conducted to determine if variation in Daphnia growth rate and fecundity when fed food of varying quality was caused by a change in feeding rate. Daphnia feeding rate increased with improved food quality, suggesting that Daphnia responds to increases in food quality, at least in part, by increasing feeding rate. We conclude that food quality can strongly affect Daphnia feeding, growth, and reproduction, thereby constraining food-web dynamics in nutrient-deficient lakes.