The piscicidal potential of water-extract mesocarp of Azardirachta indica was investigated in static biossay experiment with continuous aeration to determine its acute toxicity. The 96-hLC50 was 97.42mg/l while the threshold value was 31.12mg/l. The extract led to an initial increase in opercula ventilation rates which then decreased significantly. The computed regression equation (y) = 4.624 + 6.352 Logc. (r=0.72) and the fish exhibited respiratory distress, loss of balance and erratic swimming prior to mortality. The usefulness of A. indica as piscicidal plant is discussed.