The present study focuses on the establishment of methods for biomarker studies in freshwater and marine fish species as a basis for monitoring the extent of contamination of fisheries resources in tropical waters. Riverine catfish (Rita rita) and marine mudfish (Apocryptes bato) were given a single intraperitoneal injection with two selected inducing compounds; beta-naphthoflavone (BNF, 50 mg/kg) and a polychlorinated biphenyl mixture (Clophen A50, 20 mg/kg), and the heavy metal compound cadmium chloride (CdCl2, 1 mg/kg). Effects on cytochrome P4501A (CYP1A) were determined in post-mitochondrial supernatants (PMS) of liver at days 3 and 10 after treatment. EROD (7-ethoxyresorufin O-deethylase) activity and CYP1A protein level by indirect non-competitive enzyme-linked immunosorbent assay (ELISA) and Western blotting using a monoclonal antibody against fish CYP1A, were measured. BNF and Clophen A50 resulted in induction upto 9.5- and 5-fold, respectively, of CYP1A protein compared to control, while CdCl2 showed significant inhibition in these species. The present study examined the phase-I cytochrome P450 monooxygenase activity and response in these two tropical fish species for the first time.