1. The effects of feeding on suboptimal foods were investigated in Daphnia cucullata a zooplankton common in many types of lakes. Eleven clones of D. cucullata were collected from four lakes of varying trophic levels and fed a high (1 mg C l1) concentration of one of two diets: (i) a 1 : 9 mixture of the cryptophyte Cryptomonas pyrenoidifera with the green alga Scenedesmus obliquus; and (ii) a 1 : 9 mixture of the cryptophyte C. pyrenoidifera with the filamentous cyanobacterium Oscillatoria limnetica. 2. As expected, the diet dominated by Oscillatoria was found to be significantly poorer compared with the diet dominated by Scenedesmus. All D. cucullata clones were, however, able to achieve positive population growth rates on both diets, supporting the view that daphnids can grow and reproduce on a diet dominated by filamentous cyanobacteria. 3. Clones originating from different types of lakes differed in their responses to the two diets. On the Scenedesmus-dominated diet, clones originating from moderately eutrophic lakes had lower population growth rates than clones from hypertrophic lakes. In contrast, on the Oscillatoria-dominated diet, the clones from moderately eutrophic lakes had higher population growth rates than clones from hypertrophic lakes. Contrary to expectation, clones originating from lakes dominated by filamentous c 4. As the reactions of clones originating from same type of lake resembled each other, it is hypothesized that the life histories of D. cucullata populations are locally adapted to environmental factors other than abundance of filamentous cyanobacteria, which are correlated with trophic levels.