ABSTRACT Root tip meristems were stained to demonstrate ATPase activity by two different methods, with general agreement in localization but not specificity, and with emphasis on mitotic cells. In interphase, ATPase was localized in nucleoli and nuclear membranes, with lesser activity in the nuclear substance. In prophase, chromosomes were outlined by ATPase stain which gradually declined in intensity at prometaphase, becoming least evident in metaphase. Staining activity increased again in anaphase, and remained high in telophase. In prometaphase, anaphase and late anaphase-early telophase, the ATPase was concentrated in a fibril which appeared to coil around the chromosomes. The ATPase fibril was thinnest at metaphase and shorter and thicker at telophase. In addition, granules farmed in association with the coils of the fibril in late anaphase and early telophase. Later on, these granules may have fused and contributed to nucleolar reformation. The ATPase never localized in the chromosomal fibre nor in any other region of the spindle. RNA generally localized like ATPase, but ATPase loci were unchanged after ribonuclease (RNase) treatment. Because of certain similarities between ATPase and argentaffin localization, some relationship between the nucleolus and ATPase is suggested. A mechanochemical transducing role is postulated for the ATPase, because cytochemical properties were like those of ATPase in the A-band of myofibrillae, and because other changes in it could be correlated with chromosome movement.