Flooding of soil for 66 days decreased the rate of dry weight increment and induced stem hypertrophy and abnormal cambial growth in 3-year-old Thuja orientalis seedlings. The rate of dry weight increase of seedlings was reduced largely because of root decay and a decrease in root growth and in initiation of new roots. Diameters of the submerged portions of stems increased as a result of accelerated growth of bark as weil as xylem increment. stem diameters also increased above the water level but alm ost en tirely because of an increase in tracheid production. The xylem of submerged portions of stems was characterised by wide variations in tracheid size (with many largediameter tracheids), lack of arrangement of tracheids in orderly radial rows, and an increased number of xylem rays and large ray cells. Flooding also stimulated ethylene production in stems but did not affect formation of resin ducts in the bark and did not induce formation of compression wood. Ethylene appeared to play a role in regulating cambial growth of flooded plants.