Due to their widespread use, plastics are a major source of pollution in freshwater environments, but their effects on aquatic organisms is largely unknown. This study examined the effects of 30 µm polystyrene microplastics (MP) on the freshwater microalgae Raphidocelis subcapitata. Algal population growth increased after exposure to 250 mg/L and decreased after exposure to 500 mg/L polystyrene. The opposite trend was observed in photosynthetic parameters with a decrease up to 250 mg/L and an increase after exposure to 500 mg/L for 7 d, as compared to controls.