AbstractDistribution ofXanthomonas populisubs,populiwas studied 21 days postinoculation in highly susceptible, susceptible and tolerant poplar clones (Populus trichocarpa): Heimburger, Muhle‐Larsen and Columbia River. Bacteria were observed in the intercellular spaces of leaf trace of both clones Heimburger and Muhle‐Larsen. No bacteria were detected 21 days after inoculation (dpi) in ultrathin sections of the clone Columbia River. The direct effect of bacteria on the host cells was the expanding intercellular space of leaf traces by dissolution of the middle lamellae and cell walls, and collapse of the surrounding cells.Sixteen weeks following the inoculation a tumour developed only at the inoculation site of the clone Muhle‐Larsen; Heimburger showed longitudinal c