In modern age, Arsenic is a major environmental pollutant and exposure occurs through agricultural, environmental, medicinal and occupational sources. The toxicity of arsenic trioxide has been shown in catfish, Clarias batrachus and the data suggest that the inorganic forms of arsenic showing the highest toxicity level. Aim and Objectives: To study the complete blood count of control and arsenic trioxide affected Clarias batrachus, and provide an overview on arsenic and heavy metal toxicity. Methodology: All 50 fishes (10 in control group and 40 in experimental group) were selected for haematological studies and blood samples collected from cardiac puncture for the next 96 hours in the interval of 24 hours. Results: The result of present study indicate that the RBC, Hb, Platelets, PCV, MCV and MCH value were decreased, fluctuation in differential leucocytes count and increased in the number of WBC due to effect of arsenic trioxide. Conclusion: Haemopoietic cells and their activities were decreased due to arsenic contamination. The higher number of white blood cell counts represent some abnormalities due to physical stress, damage or infection in body tissues and leukaemia also. The present investigation indicates that arsenic trioxide exposure may affect the haematological profiles.