Just before nuclear division, the chromosomal elements within the large, highly polyploid macronucleus of I. multifiliis carry out rotational movements. Electron micrographs of cells fixed during the rotational movements show islets filled with microfilaments in various states of aggregation. Both thick (80–200 A) and thin (30–80 A) filaments occur, either as a highly dense network or as straight, in part parallel, filaments embedded in a filamentous network of lower density. Other “islets” of the macronucleus contain large and dense aggregates of filaments, sometimes with globular particles measuring 50–60 A arranged along the thick filaments, occasionally forming cross-bridges with the thinner ones. — After incubation of the cells before fixation in a contractionsolution containing 0.002 M ATP, all nuclear islets show a nearly uniform appear ance of filamentous aggregates: numerous long and thick filaments are arranged in parallel with thin filaments with which they are in some parts connected by bridges. The probable myosinoid and actinoid nature of thick and thin filaments is discussed. It is suggested that the pre-divisional intranuclear rotational movement is a mechanism to avoid aneuploidy by producing a random arrangement of replicated hereditary units prior to division.