Abstract: Studies on the influences of lindane,pirimiphos methyl,permethrin,zinc and dodecyl linear alkybenzene sulfonate(LAS)on the activity and toxicity of cholinesterase(ChE)and glutathione s-transferase(GST)in Gammarus pulex L. showed that only pirimiphos methyl caused a change in ChE activity in Gammarus,with a significant reduction in enzyme activity after²4h and 48h exposure.Both lindane and permethrin caused a change in GST activity in Gammarus,with a significant increase in enzyme activity after 48h exposure.Lindane alos caused a significant increase in GST activity after²4h exposure.Biomarkers ChE and GST were demonstrated a high degree of specificity and sensiti- vity in comparison to the lethality assay,but GST activity was less specific than ChE activity.