The extensive use of household and industrial cleaning products has led to increased environmental pressure on aquatic ecosystems, with substances like benzalkonium chloride posing potential hazards to aquatic life. This study examines the xenobiotic effects of benzalkonium chloride on the gill and muscle tissues of the freshwater fish Cyprinus carpio (common carp). Fingerlings (average weight: 4.3 ± 1.4 g; length: 5.1 ± 1.7 cm) obtained from a fish farm in Tumkur, Karnataka, were used to evaluate acute toxicity through static bioassay experiments, including determination of the 24-hour LC₅₀ value. The fish were exposed to varying concentrations of benzalkonium chloride, and behavioral responses along with mortality were monitored at 2-hour intervals. Biochemical assessments indicated significant changes in glucose, glycogen, and protein levels in both gill and muscle tissues. Glycogen and protein contents showed a notable decline, while glucose levels increased in comparison to the control group, suggesting elevated metabolic stress and energy requirements. The depletion of glycogen may reflect its utilization to meet heightened energy demands under toxic conditions, whereas reduced protein levels indicate increased protein breakdown. Histological observations revealed considerable damage to gill structures, including epithelial desquamation, necrosis (telangiectasia) of secondary lamellae, lamellar shrinkage, and hypertrophy and hyperplasia at the lamellar base. Muscle tissue analysis also demonstrated significant alterations such as nuclear proliferation, irregular fiber size, degeneration and atrophy of muscle bundles, and separation with thickening of fibers. Overall, the study indicates that benzalkonium chloride causes pronounced biochemical and histological changes in Cyprinus carpio, particularly in early life stages, emphasizing its ecological significance and the need for regulated use of such contaminants in aquatic environments.