The acute toxicity of ammonium sulphate, calcium ammonium nitrate and two different composite fertilizers [15:15:15 (NPK-1) and 25:5:10 (NPK-2)] to juvenile rainbow trout (Oncorhynchus mykiss; 0.89 +/- 0.22 g) was evaluated under semi-static conditions. The concentrations of ammonium sulphate, calcium ammonium nitrate, NPK-1 and NPK-2 that killed 50% of the rainbow trout within 96-h (96-h LC50) were 0.149, 0.259, 0.258 and 0.408 g/L, respectively. The mortality of fish was elevated with increasing concentrations of the fertilizers. The time required to kill 50% (LT50) of the rainbow trout at the highest concentrations tested was 11 h 18 min for ammonium sulphate, 12 h 23 min for calcium ammonium nitrate, 13 h 38 min for NPK-1 and 17 h 47 min for NPK-2. Ammonium sulphate was the most toxic compound to rainbow trout compared to composite fertilizers.