Keywords: The large spindle-shaped micronucleus of Paramecium bursaria contains filaments of about 60 Å diameter between numerous highly condensed chromosomes. Following treatment for several hours with 10% heavy water the nucleus becomes streched to several times its original length. The filaments, whose number increases noticibly, then appear as an aggregate, which exhibits a somewhat paracrystalline arrangement. An examination of the filament aggregate by means of a laser diffractometer yielded two diffraction lines corresponding to longitudinal periodicities of 91 - 99 and 160 -195 Å. Since both periodicities are comparable to those reported for microtubular proteins, it is assumed the filaments reported here could be related to microtubular material.