It is aimed to determine possible alterations in gill ultrastructure of Oreochromis niloticus individuals exposed to certain concentrations of Lambda-cyhalothrin technical formulation with Transmission Electron Microscope (TEM). At 7th, 14th and 21st days fish exposed to 0.29 µg/L lambda-cyhalothrin were taken from the test solution. The gills were fixed with 2.5% glutaraldehyde and post-fixed withosmium tetroxide, dehydrated with ethanol and propylene oxide, and then embedded in resin. The ultra thin sections were stained with uranyl acetate and lead, and examined with TEM. At 7th, 14th and 21st days the most common ultrastructural alteration was deformation of pillar cells. The severity of cytopathological alterations were increased with the duration of exposure.       This study was supported by TUBİTAK under grant number 114-Z-730.