Diethyl phthalate (DEP) is used as a plasticizer, a detergent base, in aerosol sprays, as a perfume binder in incense sticks and after-shave lotions. It is known to be a contaminant of freshwater and marine ecosystems. Therefore, a study was designed to determine the toxic effects of DEP on a freshwater fish, Cirrhina mrigala. The fish was treated with 25, 50, 75, and 100 ppm (w/v) DEP dissolved in acetone to determine the LC50. Positive controls were treated with acetone only. There was 100% mortality observed within 24 h in 75 and 100 ppm, and 50% mortality in 50 ppm treated fish in 72 h. Those treated at 25 ppm showed only 10% mortality within 72 h and remaining fish continued to survive. The surviving fish were treated with 25 ppm DEP once daily for 3 days with every change of water (Group III). One group was maintained as negative control in dechlorinated water (Group I) and the other group received acetone once daily for 3 days with every change of water and was used as positive control (Group II). Fish were killed by cold narcosis on an ice block and dissected to obtain liver, muscle, and brain samples; 10% homogenates in ice-cold saline were prepared. Brain and muscle acetylcholinesterase (AchE) activity was measured. Liver aspartate (AST) and alanine aminotransferase (ALT), and liver and muscle succinate dehydrogenase (SDH) alkaline and acid phosphate (ALP and ACP) were measured. There was a significant increase in liver and muscle ACP and ALP in DEP-treated fish compared with positive and negative controls. There was a significant increase in muscle SDH and liver ALT (ALT) in DEP-treated fish compared with positive and negative controls. Brain AchE level was significantly decreased in DEP-treated fish compared to positive and negative controls. These results indicate that DEP brings about significant changes in the activity of certain liver and muscle enzymes. These alterations in enzyme activity may have long-term effects on that are continuously exposed to low doses of DEP in the aquatic environment.