Objectives : This study was aimed to examine the acute toxicity assessment of two new algicides, thiazolidinediones derivatives (TD53 and TD49), which were synthesized to selectively control red tide, to the marine ecosystem. Methods : The assessment employed by a new method using Ulva pertusa Kjellman which has been recently accepted as a standard method of ISO. The toxicity was assessed by calculating the $EC_{50}$ (Effective Concentration of 50%), NOEC (No Observed Effect Concentration) and PNEC (Predicted No Effect Concentration) using acute toxicity data obtained from exposure experiments. $EC_{50}$ value of TD49 and TD53 was examined by 96-hrs exposure together with Solutol as a TD49 dispersing agent and DMSO as a TD53 solvent. Results : $EC_{50}$ value of TD53 was $1.65;{mu}M$. From the results, values of NOEC and PNEC were calculated to be $0.63;{mu}M$ and 1.65 nM, respectively. DMSO under the range of $0{sim}10;{mu}M$, which is same solvent concentration used in examining TD53, showed no toxic effect. $EC_{50}$ value of TD49 was $0.18;{mu}M$ and that of Solutol was $1.70;{mu}M$. NOEC and PNEC of TD49 were $0.08;{mu}M$ and 0.18 nM, respectively and those for Solutol were $1.25;{mu}M$ and 1.25 nM, respectively. Conclusions : From the values of NOEC, PNEC of TD53 and TD49, TD49 showed 9 times stronger toxicity than TD53. On the other hand, DMSO showed no toxicity on the Ulva pertusa Kjellman, but Solutol was found to be a considerable toxicity by itself.