Abstract: We conducted a static bioassay test to investigate the acute toxicity of compound trichloroisocyanuric acid to larvae of Penaeus semisuicatus. The concentration of TCCA in the experiment for nauplius and zoea is 0 mg · L^-1, 0.5 mg · L^-1, 1.0 mg · L^-1, 1.5 mg · L^-1, 2.0 mg · L^-1, 2.5 mg · L^-1 and 3.0 mg · L^-1; that for mysis and post larvae is 0 mg · L^-1, 1.0 mg · L^-1, 1.5 mg · L^-1, 2.0 mg · L^-1, 2.5 mg · L^-1, 3.0 mg · L^-1, 3.5 mg · L^-1 and 4.0 mg · L^-1; each concentration has 3 replicates. The results show that the 24 h LC₅₀ and 48 h LC₅₀ with 95% confidence interval for nauplius are 1.47~1.78 mg · L^-1 and 1.34~1.46 mg · L^-1, respectively;those for zoea are 1.67~1.78 mg · L^-1 and 1.23~1.36 mg · L^-1, respectively;those for mysis are 2.05~2.25 mg · L^-1 and 1.58~1.79 mg · L^-1, respectively;those for post larvae are 3.14~3.38 mg · L^-1 and 2.59~2.78 mg · L^-1, respectively. The safe concentration (SC) for nauplius, zoea, mysis and post larvae is 0.30 mg · L^-1, 0.22 mg · L^-1, 0.34 mg · L^-1 and 0.55 mg · L^-1, respectively. The results provide references for safe medication in larvae rearing for P.semisuicatus.