In order to study the toxic effects of an antifouling agent, tributyltin (TBT), thornfish (Terapon jarbua Forsskål) were exposed to 1 and 5μg/l TBT for 60 days and the morphological changes of the trunk muscle in this species were examined. Light microscopical examination revealed severe muscle atrophy and/or necrosis in the 1 and 5μg/l TBT group, vacuolation among myofibrils and the broken myofibrils in the injured white muscular tissue. The morphometric analysis showed that the relative area of vacuolation was dosage-dependent. Swelling of mitochondria and cristae degeneration were detected from injured muscle cells of the 5μg/l TBT-treated fish using a transmission electron microscope (TEM). Additionally, the relative body height significantly decreased from 30.8±0.7% (control group) to 28.0±0.8% (1μg/l TBT group) and 27.6±0.9% (5μg/l TBT group) (p＜0.05, n=10), revealing slimmer and/or thinner body shape. This study elucidated histopathologic diagnoses of muscle disorders due to TBT toxicity.