Ethrel (2-chloroethylphosphonic acid) applied in lanolin paste at concentrations of 0.4, 1.6, 6.2, or 10.8% to stems of 3-month-old Ulmus americana seedlings greatly altered stem anatomy within 41 days. Application of ethrel at 1.6% or higher concentration was followed by greatly increased bark thickness primarily as a result of an increase in the amount of phloem and intercellular spaces. Xylem increment was increased following treatment with 0.4 or 1.6% ethrel and reduced by 6.2 or 10.8% ethrel. All concentrations of ethrel increased the number of vessels, reduced vessel diameters, and induced an increase in ray width and size of the individual ray cells. Ethrel at 6.2 or 10.8% inhibited differentiation of fibres, many of which were poorly developed and contained protoplasm and nucleL Ethrel also stimulated accumulation of dark-staining organic deposits in the ray parenchyma cells, axial parenchyma cells, and immature fibres. The data indicate a role of ethylene in control of growth and anatorny of stems.