Abstract Due to global climate change, the aquatic organisms adapt to drastic temperature fluctuations and low dissolved oxygen levels in the freshwater ecosystem; however, the development of thermotolerance to extreme temperatures (32°C) and novel stresses (e.g. hypoxia) depends on a graded physiology and cellular stress response. In order to assess the effects of elevated temperature on cellular physiology, common goldfish (Carassius auratus) were purchased from local market and randomly assigned to six 20-gallon aquaria (20 fish/aquarium). All fish were acclimatized for 1-month (22°C) and exposed to control (22°C), medium (26°C), and high (32°C) temperatures for 4-week. After the experimental period, brain, liver, kidney, and gill tissues were collected and fixed in 4% paraformaldehyde. Basic parameters (e.g., condition factor, morphometry) were measured and the histological findings of gill, kidney, liver, and brain tissues were analyzed. Tissues from medium and high treatment groups (26 and 32°C) revealed cellular injury and necrotic cell death. The histological lesions of neuronal vacuolation and reduced glomerular diameter were found in brain and kidney tissues, respectively, at high treatment groups. Further studies for the confirmation of altered activity/expression of antioxidant enzymes (e.g., catalase, superoxide dismutase, and glutathione peroxidase), oxidative biomarkers (e.g., dinitrophenyl protein, nitrotyrosine protein, protein carbonyl content) and in situ TUNEL assay for cellular apoptosis will elucidate a mechanistic approach to the graded cellular stress response in goldfish.