In order to investigate the effect of the allelopathic compound pyrogallic acid on Cylindrospermopsis raciborskii F2, the impact on growth, oxidative stress and expression of the psbA, grpE, fabZ, recA, cmpA, ftsZ and cyrJ genes were studied. The results indicated significant decreases in Chl a and cell number following a 24-h incubation with 4 mg L(-1) pyrogallic acid. Additionally, malodialdehyde content, superoxide dismutase activity and catalase activity were enhanced following treatment with 2 and 4 mg L(-1) pyrogallic acid. Expressions of the genes psbA, grpE, fabZ, recA and cyrJ were significantly up-regulated following exposure to 4 mg L(-1) pyrogallic acid, while no changes were observed with concentrations of 1 or 2 mg L(-1). Expression of cmpA was significantly down-regulated following treatment with the lowest tested concentration of pyrogallic acid (1 mg L(-1)), while ftsZ was only significantly down-regulated with concentrations of 2 and 4 mg L(-1). These results suggest that photosynthesis inhibition and oxidative damage are important modes of action for the allelopathic effect of pyrogallic acid on C. raciborskii.