This study compared acute ammonia toxicity in juvenile rockfish and red sea bream. Oxygen concentration affected ammonia toxicity to aquatic animals. Without aeration, the 96-hr $LC_50$ values of $NH_4^+$and un-ionhized ammonia for juvenil rockfish(3.6g, 6.6cm) were 2.61 and 0.09 mg/l, respectively. The 3-hr $LC_50$ values of $NH_4^+$ for the size of 1.9g (5.2cm) and 3.6g (6.6cm) rockfish, and 1.0g (3.8cm) and 2.0g (5.0cm) red sea bream without aeration were 3.96, 3.94, 4.35 and 3.75 mg/l, respectively. When oxygen level was low, larger fish were more susceptible to ammonia toxicity than smaller one because of stress resulted from low oxygen. However, with aeration, the 96-hr $LC_50$ values of $NH_4^+$ for the size of 1.2g (3.9cm) and 2.3g (5.1cm) red sea bream with aeration were 3.84 and 3.90mg/l, respectively. The 6-hr $LC_50$values of $NH_4^+$ for the size of 3.1g (5.9cm) and 6.2g (7.0cm) rockfish with aeration were 3.83 and 3.94 mg/l, respectively. When oxygen level was high, larger rockfish and red sea bream were less susceptible to ammonia toxicity than smaller ones. The 6 hr-or 96 hr-$LC_50$ values of $NH_4^+$for rockfish with reduced ammonia toxicity. In comparing 96 hr-$LC_50$ values of $NH_4^+$ and un-ionized ammonia for juvenil rockfish with those for juvenil red sea bream, the values for rockfish were lower than for red sea bream. This indicates that juvenile rockfish is more susceptible to ammonia toxicity than juvenile red sea bream