Profenofos, an organophosphate insecticide is acetylcholinesterase inhibitor that has the potential to contaminate the ground water. The 96 h LC(50) value of profenofos was determined in 3-month-old fingerling common carp (Cyprinus carpio) with a body weight 1.04 +/- 0.25 g and a body length 4.25 +/- 0.75 cm at 26 +/- 1 degrees C temperature. Trimmed Spearman-Karber (TSK) software was used for the statistical analysis, which calculated the LC(50) value as 62.4 microg/L for three replicates of the assay. The behavioral responses of fish exposed to profenofos included loss of balance, moving in spiral fashion with sudden jerky movements, lying on their sides and rapid flapping of the operculum with the mouth open.