Background: The evolution of multicellular organisms must, at some point, have involved the congregating of single-celled organisms. Algal species exist that sometimes live in groups and sometimes live as single cells. Understanding the conditions that lead to algal assemblage in such cases may cast light on the selective forces that favour multicellularity. Hypothesis: Forming groups could defend algae against predation if predators are unable to engulf large-sized entities. Organisms: Three algal prey (Chlorella sorokiniana, Chlorella vulgaris, and Scenedesmus obliquus) and three predators (Ochromonas spp., Tetrahymena thermophila, and Daphnia magna). Methods: We tested the tendency to aggregate in all nine different prey–predator combinations. Results: At least two of the predators, Ochromonas and Daphnia, were significant predators because their presence decreased algal density. In all nine combinations, adding the predator species led to the formation of algal groups. In three combinations, adding merely products of the predators in the absence of the predators themselves stimulated group formation.