Histopathological changes in gills of Oreochromis sp. exposed for 96 hr with three different concentrations of clove oil, 48 mg/L, 65 mg/L and 81 mg/L were examined in this study. These concentrations were selected by 25% lower and 25% higher than LC50 value (65 mg/L). The gills were fixed in 10% FAA (formalin acetic acid) for histological observation. Severe lesions such as lamella fusion, telangiectasis and oedema were observed at 48 mg/L. At the concentration of 65 mg/L the gills exhibited the occurrence of oedema and necrosis whereas at 81 mg/L, lifting of lamella epithelium and necrosis were noticeable.