The present study is aimed at evaluating the pathological changes in various tissues of Oreochromis mossambicus exposed to the neem based pesticide achook. The LC50 value obtained after 96 hrs was 0.105 ppm and 1/5th of LC50 value i.e., 0.021 ppm was taken as sublethal concentration. The fishes were exposed to this sublethal concentration for a period of 42-days. Fishes were dissected at the end of 7th day and 42nd day of exposure and the gill, liver and intestine were collected from exposed fishes and control group. Tissues were processed and sectioned at 4µm and stained with Haematoxylin-Eosin. The histological changes observed were mild on 7th day but became severe by 42nd day. The pathological changes in gills of exposed fishes include shrunken and fused gill lamellae, inflammatory cells in the primary and secondary gill lamellae. Exposed liver showed lesions consisting of vacuolar degeneration and disruption of hepatocytes. The changes found in intestine were vacuolar degeneration and disruption of epithelial cells of villi, ruptured and shapeless villi. The histology of tissues from control fishes were normal.