The fingerling Grass carp, Ctenopharyngodon idellus, was exposed in different concentrations of meothrin nearby 24h LC-50 and its gill morphology was examined by light and electron microscopy. The results indicate that meothrin caused fish convulsions, cough, ataxia, intermittent paralysis and overturned. Morphological examination revealed severe alterations to compare with control fish. These initially consisted in hypertrophy of the secondary lamellae, and finally resulted in local telangiectasia, fusion between secondary lamellae. Ultrastructural changes in the gills showed the pillar cell system collapse and resulted in large non-tissue spaces which were invaded by leucocytes. The cytoplasm of the epithelial cells appeared many myelin figures, electron dense deposits. Chloride cells were severely damaged with a degenerated mitochondria and nuclear membrane. Higher concentration of meothrin caused a loss of adhesion between the epithelial cells, accompanied by a collapsing of the structural integrity of the primary lamellae and degenerating of epithelial and chloride cells.