Abstract: Prymnesium parvum is one of the ichthyotoxic microalgae, causing mass mortality of cultured fish.To understand the safety threshold and the poisoning patheway of P.parvum to fish, the research on the acute toxicity and the changes of superoxide dismutase (SOD) activity, catalase (CAT) activity and malondialdehyde (MDA) content of Oryzias melastigma exposed to P.parvum was carried out.The results showed that there was a dose-effect relationship between P.parvum cell density and the toxicity on O.melastigma when the cell density of P.parvum ranged from 0.9×10⁸ to 11.2×10⁸/L.The LC₅₀ in 3 h, 24 h and 48 h were 5.62×10⁸, 1.58×10⁸ and 1.44×10⁸/L, respectively.The LT₅₀ of O.melastigma exposed to the P.parvum cell density of 1.87×10⁸ and 2.8×10⁸/L were 41.75 h and 6.5 h.The maximum ineffective concentration of P.parvum was 0.9×10⁸/L, from which we speculated cell density of 9×10⁶/L could be the safety threshold of P.parvum to fish.Activity of SOD decreased when O.melastigma exposed to lower cell density of P.parvum, but it didn't show a dose-effect relationship between SOD activity of O.melastigma and cell density of P.parvum.O.melastigma exposed to higher cell densities of P.parvum had a higher activity of CAT.A higher content of MAD in O.melastigma was detected when they exposed to lower cell densities of P.Parvum, it implied that a lower cell density of P.parvum could lead to lipid peroxidation in O.melastigma, which could be one of the poisoning pathways of P.parvum to fish.