Textile is one of the most economic sectors in India. However, continuous discharge of textile waste waters loaded with a variety of synthetic dyes pose a huge threat to the surrounding ecosystems. Textile azo dyes are serious pollutants of the aquatic environment because of their environmental persistence and ability to be accumulated by aquatic organism. Here, we investigated the toxic effects of azo dye, Reactive Red 120 (RR 120) on the behavioural and some haematological parameters of the freshwater fish, Indian major carp Catla catla fingerlings. The fingerlings were procured and acclimatized in the laboratory condition for 15 days according to APHA. The LC50 value of RR 120 for C. catla, fingerlings was 35 mg L-1 for 96 h. Catla fish showed an abnormal behavioural change against azo dye RR 120 intoxication such as opercular movement, sluggish, lethargic and abnormal swimming, and loss of muscular tetany. Then, they were grouped in to 10 (10 in each). Among this one was considered as control and other groups of fingerlings were exposed to sub lethal concentrations (0.35, 0.7 and 3.5 mg L-1) in triplicate of RR 120 for 30 days. At fixed interval (10, 20,30 days) after the exposure blood samples were collected and were analyzed. Results obtained from this study show significantly RBC, Hb and PCV value exhibited significant decreased whereas, WBC, MCHC, MCV and MCH were increased in fish as compared control in all RR 120 exposed fingerlings. The present study clearly indicates the toxic effects of RR 120 even at sublethal concentrations of exposed fingerlings.