A substance produced extracellularly by a marine bacterium, Pseudomonas C55a-2 strain, and possessing algal-killing activity against a diatom, Chaetoceros ceratosporum, was isolated from the culture supernatant of the bacterium in order to identify its chemical structure. The algicidal substance extracted with ethyl acetate from the culture supernatant was purified by using Sep-Pak treatment, reverse phase column chromatography with Sephadex LH-20, and high performance liquid chromatography (HPLC) with Mightysil RP-18 GP. The purified substance was identified as 2,3-indolinedione (isatin) (molecular weight 147) based on hydrogen-nuclear magnetic resonance (H-NMR) and gas chromatograph-mass spectrometry (GC-MS) analysis. Among artificial synthetic compounds examined for algicidal activity against Chaetoceros cells on the double-layer plates, 2,3-indolinedione (isatin) appeared the most effective and indoline showed relatively less activity than isatin.