The 96-h LC50 (median lethal concentration, LC50) tests were conducted using four different sizes of yellow catfish Pelteobagrus fulvidraco to provide primary information on the sensitivity of this species to elevated ammonia and/or nitrite, and to determine if the sensitivity is mediated by size under the same conditions. The results showed that 96-h LC50 of fish weighing 0.034 ± 0.002, 0.296 ± 0.049, 3.52 ± 0.95 and 32.96 ± 5.75 g to total ammonia nitrogen-N was 24.96, 35.85, 47.44 and 68.79 mg L−1, respectively; un-ionized ammonia nitrogen-N was 0.34, 0.49, 0.65 and 0.94 mg L−1 in test conditions of pH 7.42 and 23°C; and that nitrite nitrogen-N was 69.06, 97.23, 133.61 and 196.05 mg L−1 in test conditions of pH 7.58 and 23°C, respectively. The NOEL (No Observable Effect Level) of fish (body weight from 0.03 to 30 g) to ammonia and nitrite was 2.25–6.22 mg L−1 total ammonia nitrogen-N, 0.03–0.10 mg L−1 un-ionized ammonia nitrogen-N in test conditions of pH 7.42 and 23°C, and 6.27–17.68 mg L−1 nitrite nitrogen-N in test conditions of pH 7.58 and 23°C, respectively. These results indicate that the susceptibility of this fish to total ammonia or nitrite was reduced with increasing size, and that a dose-dependent relationship might exist between them. The 96-h LC50 and NOEL of different sizes of fish to total ammonia, un-ionized ammonia and nitrite would be important to know for water quality standards in yellow catfish aquaculture.