Interspecific hybridization is a common phenomenon among Daphnia species (Crustacea: Anomopoda); interspecific hybrids and parental species have been shown to be ecologically differentiated and often co-occur in the same lake. Food quantity, temperature, and the level of predation (by juvenile fish) have been identified as the main environmental factors determining the fitness of Daphnia taxa. Here we tested another environmental factor, food quality, which is known to shape fitness in Daphnia. We conducted life-history experiments with clones of Daphnia galeata, Daphnia cucullata, and their interspecific hybrids and measured fitness-related life-history traits at two food quality conditions (phosphorus [P]-rich and P-limited algae). D. galeata x cucullata hybrids show highest fitness values in some traits at low food quality conditions, relative to the parental species, whereas D. galeata was superior in P-rich conditions. These results, based on single-clone life-history studies, were confirmed by a multiclone experiment. Large natural variation in food quality and the observed differential response of clones and taxa to P variation indicates that variation in food quality represents an additional factor, besides fish predation and food quantity, explaining hybrid maintenance. © 2007, by the American Society of Limnology and Oceanography, Inc.