The toxicity of Poterioochromonas to Daphnia magna was investigated at different food (Scenedesmus acutus) levels. Poterioochromonas alone of 0.4-20 mg C L(-1) was not acutely toxic to D. magna, but did not support D. magna growth, either. When fed mixed diets (2 mg C L(-1) in total), D. magna's survival and reproduction were significantly depressed when Poterioochromonas comprised above 50%, likely due to the inhibition of food ingestion. Large juveniles were less sensitive to poor quality food than neonates. Therefore, Poterioochromonas may affect D. magna living to various extents depending on its concentration, age structure of D. magna populations and availability of other food.