Transmission electron microscopy was used to study the effects of the terpenoid phytoalexins desoxyhemigossypol (dHG) and desoxymethoxyhemigossypol (dMHG) on the plasmalemma of conidia of the defoliating strain (V-44) of Verticillium dahliae. Extensive rupture of the plasmalemma occurred after exposure of conidia to 15 Mg/ml of dHG for 4 hrs. Similar treatment of conidia with dMHG at 30 Mg/ml caused only scattered swelling of the plasmalemma. Potassium ion leakage was an additional indicator of plasmalemma damage by dHG and dMHG. Potassium losses equal to about 80% of the potassium losses in boiled controls occurred after 4 hr. exposure of conidia to 25 and 45 Mg/ml of dHG and dMHG, respectively. Death of all propagules also was observed after 4 hr. exposure of V-44 conidia to both phytoalexins at these concentrations. Thus, the potassium ion leakage substantiates that both dHG and dMHG can cause significant damage to the plasmalemma of V. dahliae at these concentrations.