Abstract: Microplastics (MPs) contamination represents a dramatic environmental problem threatening both freshwater and marine organisms.Although several studies have highlighted the presence of MPs in aquatic environments, the information regarding their toxicity towards freshwater organisms is still scant.In this paper, Daphnia magna, a freshwater cladoceran, was selected as the test organism in this study.3 sizes of polyethylene (PE) particles, including 2 μm, 20 μm and 50 μm, were used to investigate their acute toxic effects according to OECD guideline for water flea acute immobilisation test.The results showed that all three sizes of PE particles could be ingested by water flea and accumulated in their guts, which caused the immobilization and may affect their lipid metabolism.PE particles in water could adhere to the body of animals, affecting their moving and feeding.In the concentration range of 5~80 mg/L, ingestion of 2 μm PE particles led to immobilisation increasing with dose and time with an EC₅₀ of 50.86 mg/L after 96 h exposure; for 20 μm and 50 μm PE particles, however, the immobilization appeared an inverted U-shaped curve with the dose increasing.After 48 h exposure, the LOEC values of the 3 sizes of PE particles were 60, 20 and 5 mg/L, respectively.It is indicated that particle size is one of the important factors affecting the ingestion and accumulation of MPs in water flea.These results underline that MPs can induce adverse effects in freshwater zooplankton and potentially impact the function of the ecosystem.