Helminthosporol is one of the natural sesquiterpenoid toxins isolated and identified in the culture medium of the phytopathogenic ascomycete fungus Cochliobolus sativus. The effect of this phytotoxin was investigated on enzymatic activities, electron and ion transport in mitochondria, chloroplasts, and microsomes of plant. The results indicate that helminthosporol drastically affects the membrane permeability of these organelles to protons and substrate anions, inhibiting the mitochondrial oxidative phosphorylation, the photophosphorylation in chloroplasts, and the proton pumping across the cell plasma membrane. The 1,3-beta-glucan synthase activity, involved in defense mechanisms of plant cells against stress and damage, e.g., during pathogen attack, was also strongly inhibited by the toxin.