The vascular system of leaves of chrysanthemum plants inoculated withVerticillium dahliaewas invaded by mycelium before the appearance of visible wilt symptoms. After flower buds appeared a rapid rise in the number of propagules of fungus in the leaf was followed by a rise in severity of visible wilt symptom expression and conductivity of leaf exudate. Those portions of the leaf with the most advanced stages of wilt contained the greatest amounts of fungus. It is suggested that the mycelium within the leaf makes a significant contribution to the development of symptoms of wilt.