The fresh water field crab, Spiralothelphusa hydrodroma is an important food source in parts of South India. The impact of cypermethrin on the variations in the enzyme activity of gills and hemolymph of the experimental female crab S.hydrodroma was carried out. The toxicity of the cypermethrin on the crab was estimated using LC₅₀. Quantitative enzymatic studies of lactate dehydrogenase (LDH), succinate dehydrogenase (SDH), acid phosphatase (ACP) and alkaline phosphatase (ALP) in the gills and hemolymph was undertaken.