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Freshwater green algae, Chlorella, have heavy cell walls and their size usually exceeds the lower limits of limb size of herbivorous Daphnia (Cladocera). According to the optimal foraging theory, we speculated that Daphnia would graze more exposed and relatively large Clamydomonas rather than Chlorella, and this process would lead to small-sized Chlorella becoming a superior competitor in the presence of Daphnia. We used Daphnia magna, Clamydomonas sajao and Chlorella pyrenoidosa to test this hypothesis. Our grazing experiments showed that Daphnia preferred C. sajao to C. pyrenoidosa, regardless of the concentration and relative abundance of these two algae. The decrease in relative abundance of high-quality Clamydomonas in Clamydomonas-Chlorella assemblages did not diminish the grazing efficiency of Daphnia on this algal species, but increased selectivity of low-quality Chlorella. However, when the concentration of Clamydomonas was extremely high, the grazing of Daphnia on Clamydomonas decreased. In competition experiments, we observed that the presence of Clamydomonas restrained the growth potential of Chlorella; however, the introduction of herbivorous Daphnia into the competing environment weakened this influence and to some extent enhanced the growth ability of Chlorella. Moreover, we also observed that the intensity of herbivory, imposed by different densities of Daphnia, had an obvious influence on the competition outcome between Clamydomonas and Chlorella. At the highest intensity of herbivory (10 Daphnia), C. sajao was eliminated from the culture medium whereas C. pyrenoidosa could persist, but at low cell density. [ABSTRACT FROM AUTHOR]