Abstract: The toxic effects of azadirachtin on fishes were studied. The results showed that azadirachtin had lower acute toxicity to Brochydaino rerio. The LC50 of Brochydaino rerio by azadirachtin after tested 48 h and 96 h was 41.9 and 23.3 mg/L,respectively. The safe concentration to Brochydaino rerio was 2.3 mg/L. The micronucleus test of carp with azadirachtin indicated that azadirachtin had no mutation activity to the fishes, so azadirachtin could be considered non-mutagen. At the same time, azadirachtin was used in control the parasitic diseases on fishes which caused by Trichodina sp.. During the safe concentration to fishes, the better effects on the controlling of parasitic diseases were achieved and there was a relationship between the efficacy and the concentration of azadirachtin. The results also showed that azadirachtin had a good prospect to be used as an agent for controlling the parasitic diseases of fishes.