Aim: To evaluate the impact of nonylphenol on the modulation of testicular activity of catfish Heteropneustes fossilis as well as its impact on haematological and antioxidative parameters simultaneously. Methodology: Sexually matured male catfish were divided into three sub groups: first group as control, second as vehicle control and third group exposed to 80 µg l-1 4-Nonylphenol (4-NP) for 21 days. After 21 days, testis and blood were collected for assessment of histology, haematology, oxidative stress and steroid hormone profile. Results: Present study showed that 4-Nonylphenol elicited significant effect on the testicular activity by decreasing RBC and WBC count, causing alterations in RBC morphology and induction of micronucleus in RBC of catfish. Further exposure decreased CAT (F = 34.88; P>0.001), SOD (F = 246.87; P < 0.001), GPX (F = 948.88; P < 0.001), and increased H2O2 (F = 1813.71; P < 0.001) and LPO (F = 105651; P>0.001) activities in the testis. There was significant decrease in GSI compared to control group. Histopathology of testis showed abnormalities in the testicular architecture. Estradiol-17β level significantly increased showing 4-NP as estrogenic compound also the testosterone and progesterone levels significantly decreased (F Steroids = 590.61; F Dose = 37.376; F Steroids X Dose = 142.606; P < 0.001). Interpretation: Nonylphenol exposure resulted in the impairment of male reproductive system of catfish by modulating its activity at multiple levels and also induced cytotoxicity in blood.