Abstract Background The present study was designed to determine the median lethal concentration (96-h LC50) of triclosan (TCS) and to investigate the effect of a sublethal concentrations of TCS on the activities of glutamic oxaloacetic transaminases (GOT), glutamic pyruvic transaminases (GPT), and glutathione S-transferase (GST) in the gill, liver, and muscle of an Indian major carp Catla catla. Results In this study, static renewal procedure was followed to determine the 96 LC50 value. The results indicated that the 96-h LC50 of TCS for C. catla was 0.36 mg L−1. 1/10th, 1/25th, and 1/50th of 96-h LC50 value (0.036, 0.014, 0.007 mg L−1) were selected for sublethal studies, and the fish were examined every 5 days for 20 days. The results indicated that TCS exposure to fish at three different sublethal concentrations significantly (p