Effects of lethal and sublethal concentrations 15, 20, 26 and 35 mug/l of zeta cypermethrin on the gills of common guppies (L. reticulatus) were examined by light microscopy during 96 h exposure. The most common changes at all doses of zeta cypermethrin are the lifting of epithelial layer from gill lamellae and some necrosis. Besides the exudation, hyperplasia and the shortening of secondary lamellae were other histopathological effects. The 96 h LC50 value for zeta cypermethrin to the L. reticulatus was determined 21.35 mug/l. (C) 2003 Published by Elsevier B.V.