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Not AvailableArgulosis is a serious parasitic disease in aquaculture as well as in the ornamental fish rearing. However, phytotherapy, especially employing neem and its derivative products, is being widely used to combat the pa- thogens in aquaculture. The present study was conducted to screen various extracts of neem, Azadirachta indica leaf prepared using organic and aqueous solvents of concentration ranging from 1.0 to 3.5 g L−1 for its anti- parasitic effect against adult Argulus japonicus in goldfish, Carassius auratus under in vitro and in vivo conditions for 6 and 72 h, respectively. Under in vitro condition for 6 h, antiparasitic efficacy of neem leaf extracts prepared in different solvents such as ethanol, methanol, chloroform and acetone at a concentration of 2.0 g L−1 was found to be 100, 75, 66.66 and 50%, respectively. However, aqueous extracts at a concentration of 3.5 g L−1 showed 100% efficacy. Further, the estimated 6 h median effective concentration (EC50) of ethanol, methanol, chloroform, acetone and aqueous extracts of neem leaf for A. japonicus was 1.056, 1.554, 1.877, 2.075 and 2.113 g L−1, respectively. Compared to other extracts, ethanolic extract of A. indica showed significantly high antiparasitic efficacy. However, as the estimated value of 6 h median lethal concentration (LC50) of ethanolic extract for C. auratus was 1.11 g L−1 and found to be proximate to EC50 value, application of the extract under in vivo condition will not be safe. In the bioassay, the 72 h LC50 of aqueous extract for C. auratus was found to be 3.28 g L−1. Notably, under in vivo condition, the estimated 72 h EC50 value of aqueous extract against A. japo- nicus was 1.831 g L−1 and calculated therapeutic index was 1.79. The result suggests that under in vitro con- dition, the ethanolic and aqueous extract of neem leaf solutions can be used as a potential agent for controlling adult A. japonicus. However, aqueous extract only can be applied under in vivo condition.Not Availabl