Abstract Ring width, radial cell diameter, number of cells, and percentage of vessel-, fiber-, and parenchyma-formed wood were observed at eight heights in the stems of five 30-year-old yellow-poplar ( Liriodendron tulipifera L.). Number of cells but not cell size was associated with ring width. Stems with largest diameter cells in 1949 tended to have the largest diameter cells in 1964. This was not so for ring width. Despite changes in number of cells and cell size, the relative amounts of vessel- and fiber-formed wood remained approximately 22 and 76 percent, respectively. One to 6 cells of terminal parenchyma accounted for the remaining 2 percent of ring width. Mean radial diameter of vessels and fibers increased from top to bottom along the sheath of wood formed during a given year. Tapering of vessel diameter along the stem may be important in understanding water movement velocities in tree stems. Forest Sci. 21:23-33.