Subchronic toxicity of composite inorganic fertilizers such as ammonium sulfate [fertilizer A; (NH₄)₂SO₄; 21% NH₄-N)], composite fertilizer 15-15-15 (fertilizer B; commercial formulation: 15% NH₄-N, 15% phosphorus, and 15% potassium oxide), and composite fertilizer 25-5-10 (fertilizer C; commercial formulation: 25% NH₄-N, 5% phosphorus, and 10% potassium oxide) on the skin, liver, kidney, pancreas, and gills of the juvenile rainbow trout (Oncorhynchus mykiss) was studied in two-week toxicity tests under static-renewal test conditions. Fish exposed to sublethal concentrations of fertilizers did not show any behavioral abnormality compared to control groups. Histological lesions were observed in skin, gills, liver, pancreas, and trunk kidney of the fish. In the epidermis, degenerated/vacuolated epithelial cells, microcystic dilatations, and intracellular edema of mucus cell were observed. Liver had swollen and degenerated hepatocytes without losing adenoid structure. Hematopoietic tissues had necrosis and vacuolar degeneration on proximal tubules of the kidney. In order, the most affected organs were skin, liver, and kidney.