SUMMARYFlammulina velutipesfruitbodies were grown on nutrient-supplemented poplar sawdust. Cells in the middle and tip of the stipe became longer with increasing fruitbody length, but cells at the base ceased to elongate when fruitbodies reached 2–3 cm. Cell width also increased in elongating stipes. Elongation of cells of the middle region accelerated at the onset of rapid stipe growth, continued at maximum rate (110μm/day) for three-quarters of that phase, and had almost ceased when the rate of stipe growth declined. Cells in the apical region of the stipe elongated at only about one-seventh the rate of the middle cells during rapid stipe elongation. The stipe growth zone reached its maximum length during the latter phase of rapid stipe elongation, and then became shorter and restricted to the upper stipe. Cell elongation in the middle and upper middle stipe regions accounted for most of the stipe elongation but this was accompanied by cell divisions. The number of stipe cells in a row almost quadrupled between the primordium stage and the onset of rapid elongation and increased by 40% thereafter.Stipe and pileus cells were usually binucleate in primordia. Subsequently, cells of the stipe and pileus context became multinucleate but those of the lamellar trama remained binucleate. The number of nuclei in stipe cells almost doubled during early fruitbody development, and increased to a mean of 9.1-9.2 in the middle and tip cells during rapid stipe elongation. In basal cells the nuclear number reached its maximum before the onset of rapid growth. No correlation was found between nuclear number and cell size.