In the present study, the effect of salinity was studied on the toxicity of cypermethrin. A. sophiae as euryhaline species is able to tolerate salinities from 0 to 14 ppt salinity. The purpose of this study was to determine the effect of salinity on its vulnerability to agricultural pesticides. Specimens were exposed to a concentration 0.02 μg L-1 cypermethrin and two levels of salinity (0 and 14 ppt) for 14 days in laboratory conditions. The gill tissue was changed significantly with different levels of salinity. The most changes were as follow; hyperplasia of epithelial cells of secondary lamellae, hyperplasia of the mucous cell of primary lamellae, epithelial lifting, bending of secondary lamellae, scaling of epithelial secondary lamellae, pilar cell hypertrophy, fusion, shortening of the secondary lamellae and aneurism. The results showed that the sensitivity to cypermethrin decreased in lower level of salinity and its vulnerability to cypermethrin was different in various levels of salinity. Therefore, A. sophiae may not be a suitable indicator for measurement of the presence of cypermethrin in the environment.