This study evaluated acute nitrate nitrogen (NO<SUB>3-</SUB>N) toxicity in juvenile blackhead seabream Acanthopagrus schlegelii. Seventy juveniles (Trial A, 7.1±0.6 g) and nine juveniles (Trial B, 71.3±3.5 g) per 70 L tank were exposed to NO<SUB>3-</SUB>N concentrations of 0, 500, 1500, 2500, 3500, and 4500 mg/L and 0, 600, 1200, 1800, 2400, and 3000 mg/L, respectively, in triplicate for 7 days. In Trial A, all fish exposed to 3500 and 4500 mg NO<SUB>3-</SUB>N/L died within 48 h; in Trial B, all fish exposed to 3000 NO<SUB>3-</SUB>N/L died after 120 h. The 96 LC<SUB>50</SUB> and 168 LC<SUB>50</SUB> were 2505 and 1806 mg NO<SUB>3-</SUB>N/L, respectively, in Trial A, and 2663 and 2377 mg NO<SUB>3-</SUB>N/L in Trial B. Large juveniles were more resistant to NO<SUB>3-</SUB>N than small juveniles. The results of acute NO<SUB>3-</SUB>N toxicity studies provide important data for subsequent chronic toxicity studies.