The effects of the sublethal concentration (0.012%) of Congo Red on Heteropneustes fossilis were studied after 30 days exposure. The RBC count haemoglobin (Hb)% and PCV decreased significantly. The total WBC count, MCV, MCH, and MCHC showed a significant increase. Serum calcium, serum cholesterol and blood urea nitrogen (BUN) were significantly elevated, whereas serum phosphorus was significantly reduced. The activities of serum alkaline phosphatase (AlPase), acid phosphatase (AcPase). RNase, GOT, GPT and amylase were also significantly elevated. The possible reasons for these changes are discussed.