ABSTRACT: We investigated feeding of natural populations of red-tide algae, heterotrophic nanoflagellates (HNFs), and ciliates (<30 µm in cell length) on natural populations of marine bacteria (mostly heterotrophic bacteria) in diverse Korean waters (Masan Bay, Jinhae Bay, Shiwha Bay, Keum River estuary, and the open coastal waters off Yeosu) during red tides in 2004 to 2005. To explore the functional responses of the dominant red-tide algae to bacteria, we also measured the ingestion rates of the dinoflagellates Cochlodinium polykrikoides, Heterocapsa rotundata, H. triquetra, and Prorocentrum minimum as well as the raphidophytes Chattonella ovata and Heterosigma akashiwo as a function of bacterial concentration in the laboratory. During red tides, ingestion rates of the red-tide algae C. polykrikoides, H. rotundata, H. triquetra, H. akashiwo, P. minimum, and P. triestinum on natural populations of bacteria in Korean waters (1.2 to 20.6 cells alga^–1 h^–1) were comparable to those of co-occurring HNFs (0.7 to 39.4 cells HNF^–1 h^–1), but much lower than those of co-occurring ciliates (15 to 713 cells ciliate^–1 h^–1). However, the combined grazing coefficients attributable to the dominant algal predators on natural populations of bacteria during red tides (0.04 to 1.71 d^–1) were usually higher than those attributable to co-occurring HNFs (0.01 to 0.20 d^–1) or ciliates (0.00 to 0.72 d^–1). With increasing mean prey concentration, the ingestion rates of C. polykrikoides, H. rotundata, H. triquetra, P. minimum, C. ovata, and H. akashiwo on bacteria rapidly increased at prey concentrations of ca. 5 to 10 × 10⁶ cells ml^–1, and slowly increased or became saturated at higher prey concentrations. The maximum ingestion and clearance rates of C. polykrikoides, H. rotundata, H. triquetra, P. minimum, C. ovata, and H. akashiwo on bacteria (6 to 25 cells alga^–1 h^–1 and 1.0 to 4.5 nl alga^–1 h^–1) were comparable to those so far reported in the literature of HNFs on bacteria. The results of the present study suggest that, potentially, red-tide algae can have a considerable grazing impact on populations of bacteria during red tides and are sometimes the most effective protistan predators of marine bacteria.