The aim of this study was to investigate the mechanism of inhibitory action of dibutyl phthalate (DBP) on red tide algae Gymnodinium breve. The effects of DBP on malonaldehyde, subcellular structure and superoxide dismutase (SOD) isoforms were investigated. The results showed that MDA accumulated in the algae cell under DBP exposure, and for the 3 mg x L(-1) DBP treated algae culture a peak value of 0.34 micromol x (10(9) cells) (-1) occurred at 72 h, which was about 2. 3 times than that of the control. TEM pictures showed the disruption of DBP on the subcellular structure of G. breve. A morphological phenomenon appeared that the algae cell was commonly found small tubules or apical parts around the cell membrane, and almost all normal cell organelles were indistinguishable finally. The activity of CuZn-SOD (main cytoplast located isoform with little in cloroplast) under DBP exposure was higher than that of the control, and no significant difference was observed on Fe-SOD (chloroplast located isoform) activity, but for the Mn-SOD (mitochondrial isoform), the activity was significantly inhibited. These results indicated that DBP might inhibit the algae growth from the plasma membrane and the mitochondria, resulting in oxidative damage in algae cell and a final death. This paper will give a theoretical support to the practical usage of the allelochemical on red tide algae.