We investigated the antialgal effects of several bacterial and ciliate bio-agents, alone and in combination, and examined their ability to decrease the density of the warm-weather cyanobacterium, Microcystis aeruginosa, and the cold-weather centric diatom, Stephanodiscus hantzschii. Our results indicate that Microcystis aeruginosa density was effectively suppressed by the bacterium Streptomyces neyagawensis, and the heterotrich ciliate S. roeselii, both of which were isolated from the sediment and water of eutrophic Pal’tang riverine, Korea. Co-treatment with both of these bio-agents was less effective, leading to an increase in algal density that may suggest an antagonistic relationship between the bacteria and ciliate populations. Further, we found that Stephanodiscus hantzschii density was effectively suppressed by the bacterium, Pseudomonas putida, and by the S. roeselii strain mentioned above. In contrast to the above case, co-treatment with these two bio-agents was more effective than treatment with each alone, maybe a synergism. Our results suggest that co-treatment Microcystis with S. roeselii and S. neyagawensis is less effective than treatment with each agent alone, whereas co-treatment of Stephanodiscus with P. putida and S. roeselii is more effective than treatment with either agent alone.