<p><i>Trichogaster fasciata</i> is widely distributed in parts of Asia, particularly in the northeastern region of India. This species is highly susceptible to environmental contaminants, including agricultural pesticides. The present study investigated the acute toxicity of butachlor, a commonly used herbicide in Assam, on adult <i>T. fasciata.</i> A 96-h LC₅₀ test revealed that butachlor is highly toxic to fish, with a lethal concentration (LC₅₀) of 0.58&#xa0;ppm. Additionally, sublethal concentrations (1/10th, 1/20th, and 1/30th of the LC₅₀) were administered to assess behavioural changes over a 96-h exposure period. Fish exposed to these sublethal concentrations presented significant behavioural alterations, including increased swimming activity, jumping, colouration, and increased opercular movements. Proximate composition analysis on the 10th and 21st days revealed significant changes (<i>p</i> &lt; 0.001) in protein, crude fat, ash and carbohydrate contents in the treated groups in a dose- and time-dependent manner. Compared with those of the control, the protein and crude fat contents significantly decreased, and the ash and carbohydrate contents significantly increased with increasing concentration and time. These findings raise concerns about the ecological impacts of butachlor in aquatic environments and highlight the need for further studies on its long-term effects on aquatic biodiversity in regions where this pesticide is frequently used.</p>