A thiazolidinedione derivative, TD49 with the highly selective algicide to red tide was newly synthesized and its acute toxicity was examined in order to evaluate the effect on aquatic ecosystems of coast. Major three species having important role in the food chain of marine ecosystem, such as Skeletonema costatum of microalgae, Daphnia magna of crustacea, Paralichthys olivaceus of flatfish fingerling were employed for the acute toxicity assessment. $EC_50$ or $LC_50$ as the assessment criterion was investigated to each specie, and NOEC (No Observed Effect Concentration) and PNEC (Predicted No Effect Concentration) from most sensitive specie to toxicity of TD49 were further calculated. $EC_50$ of S. costatum in 96-hour, $EC_50$ of D. magna in 48-hour, and $LC_50$ of P. olivaceus in 72-hour for TD49 were $0.34;{mu}M$, $0.68;{mu}M$, and $0.58;{mu}M$, respectively. NOEC from the results of S. costatum was estimated to be $0.20;{mu}M$ and PNEC was estimated as 3.40 nM by applying factor value of 100 to $EC_50$ $0.34;{mu}M$ of S. costatum. In addition, it was revealed that Solutol used as the dispersing agent of TD49 had very little toxic influence under the concentration range of $0{sim}0.4;{mu}M$ used in TD49 toxicity experiment. Although the estimated concentration of TD49 that will be sprayed onto the coastal field for the algicide is higher than NOEC value, it is considered that the spraying concentration would not be a considerable problem due to a dilution effect by tide at the opened coast.