Argulosis is currently one among the serious health issues in aquaculture worldwide, cause significant fish health problem and economic loss. Various chemotherapeutics and phytotherapeutics has been used successfully to treat the adult stage of Argulus sp. however, no detailed study have been conducted to treat the different larval stages of the parasite. Since the parasite is prolific breeder, treating its larval stages could bring significant reduction of Argulus from the aquaculture system. In this aspect the present study was carried out to evaluate the antiparasitic efficacy (AE) of ethanol and methanol extract of neem (Azadirachta indica) leaf on hatchability of eggs, copepodid and adult stages of Argulus japonicus under in vitro condition. The eggs and copepodid stages were exposed to ethanol and methanol extract of neem leaf at concentration of 1.0 to 1.5 g L-1 for the duration of 15 days and 6 h respectively. The adult A. japonicus were exposed to various concentrations ranging from 1.0 to 2.0 g L-1of ethanol and methanol extract for 6 h. The control group containing 2 % DMSO was maintained for all the life stages of A. japonicus during each of the experiment. The results showed a significant reduction in cumulative hatching percentage of A. japonicus eggs exposed to ethanolic (8-13 %) and methanolic (12-17 %) extract of neem leaf at the concentration of 1.5 g L-1 in comparison to control group (50-73 %). The AE of ethanolic and methanolic extract of neem leaf against copepodid stage was found to be 100 % and 91.66 % respectively on 6 h at 1.5 g L-1. Further significant lower median effective concentration (EC50) was observed for ethanolic extract (0.92 g L-1) in comparison to methanolic extract (1.11 g L-1) for copepodid stage. The AE of ethanolic and methanol extract against adult A. japonicus was found to be 100 % and 75 % at 2.0 g L-1 in 6 h with EC50 of 1.056, 1.554 g L-1 respectively. The histological study of treated eggs and copepodid stage with ethanolic extract of neem leaf at 1.5 g L-1 showed deformed embryo development with undifferentiated mass of cells inside the eggs whereas, in copepodid stages, severe degenerative changes in gills (branchial region) and damage in the eye cells were observed. In the present study, ethanolic extract of neem leaf was found to be more effective against all the life stages of A. japonicus in comparison to methanolic extract. Therefore, the results obtained in this study shows the potential of ethanolic extract neem leaf against A. japonicus under in vitro condition; and this may warrant further in vivo research to evaluate the efficacy of the extract under farming condition.