Fipronil, an insecticide for crop protection with good selectivity between insects and mammals, are toxic to plants and animals in wild. In this paper, the decay patterns of Fipronil in 3 kinds of arable layer soil in Hainan province were studied by means of simulated rain water leaching use tap water on the earthworms and duckweeds, respectively. The results showed that decays of Fipronil in different soils were high related with organic matter content in soil. The death rates of earthworms

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... e 10% in up layer of clay loam (rice land) which organic matter content was 14.84 g/kg. After leaching for 14 days, Fipronil flowed into lower layer, death rates of earthworms was increased to 24%. The decreasing of death rate of earthworms were found in lower layer of three soils suggested that little Fipronil flowed into lower soils. Leaching water collected at 1 h from three soils was poisoned to cultured duckweeds which reduce chlorophyll contents and the maximal efficiency of PSII photochemistry (Fv/Fm). These studies confirmed that Fipronil was combined with organic matter at surface soil. It could not decay easily in field and will influence the crop which planted in Hainan province for long periods.