The damaging effects of fusaric acid (FA), a fungal toxin produced by Fusarium oxysporum, on cucumber seedlings were investigated in a greenhouse experiment. The accumulation of red ink was introduced as damage determination, the vines and mesophyll cells of the plants treated with high concentration of FA were acutely stained to a deep red colour, and the quantity of red ink in the shoots and roots was significantly increased. The leaf plasma membrane H+-ATPase was significantly inhibited after treatment with FA. Moreover, transmission electron microscopy and electrolyte leakage experiments revealed severe FA-induced injury to leaf cell membranes. The membrane injury and wilt in the leaves of FA-treated plants disturbed the water status, and the leaf water potential was significantly decreased. The present results suggested that FA inhibits the leaf plasma membrane H+-ATPase and reduce the cell membrane integrity of cucumber seedlings, thus leading to leaf wilting and a reduction of the leaf water potential.