Plant derived immunostimulants are a promising alternative to chemotherapeutics and also perhaps vaccines. In the present study,we examined the immunostimulating properties of aqueous leaf extract of Phyllanthus niruri, an Indian traditional medicinal herb, on neutrophil activation and antibody response of Oreochromis mossambicus (Peters). Serial ten-fold diluted doses of P. niruri ranging from 0.002 mg to 20 mg were administered to two groups of O. mossambicus (n=8). One group of fishes was administered with sheep red blood cells and the primary and secondary antibody responses were estimated using direct haemagglutination assay. The other group of fishes was administered heat-aggregated BSA to assess the ability of plant extract to elicit neutrophil activation. Our results indicate a significant enhancement of both neutrophil activation and antibody response. Among the various doses tested,fishes administered 20 mg/kg body weight caused the maximal enhancement of both primary and secondary antibody response and 0.002 mg/kg showed higher neutrophil activation compared to that of the control group. This short study indicates that aqueous leaf extract of P. niruri has the potential to be used as an immunostimulant and after confirming its immunostimulatory properties by a battery of tests on other nonspecific and specific parameters and disease-protective property by challenging the fish with virulent fish pathogens, it can be used either as a routine feed supplement to activate the immune system of farmed fishes or as an adjuvant to enhance the efficacy of vaccines.