A temperature-dependent xylem occlusion was found in cut chrysanthemum stems (Dendranthema grandiflora, cv. Viking) which were placed for 24 h in air at 5oC prior to vase life evaluation. The response was inhibited by a 5-h treatment, prior to placement in air, with aqueous solutions at low initial pH or solutions containing near-neutral antioxidants (n-propylgallate, phloroglucinol, butylated hydroxytoluene). Bacteria are known to occlude stems, but the occlusion was not related to bacterial counts in the stem ends. The number of cavitations in the xylem conduits, detected by ultrasonic acoustic emission, remained low during the storage treatment at high ambient relative humidity. The uptake of air into the stem ends ceased within 20 min whereas the occlusion developed only after several hours, showing that aspired air was not the sole cause. A xylem blockage was also found in stems placed in water directly after cutting. In these flowers, treatments with anti-oxidants delayed the occlusion, but did not affect the number of bacteria in the stem ends. The onset of xylem cavitation occurred after the occlusion. The results suggest that the stem forms a xylem blockage both during dry storage and in stems directly placed in water. The blockage apparently involves oxidative reactions.