The effects of colchicine on extranuclear microtubules associated with the macronucleus of Paramecium bursaria were studied to determine the possible role that these microtubules play in controlling the shape of the macronucleus. In the course of this study, the ultrastructure of the nuclear events of binary fission in control cells was also studied.During interphase in control cells, the micronucleus contains randomly distributed clumps of condensed chromatin and microtubular fragments. Throughout mitosis the nuclear envelope remains intact. During micronuclear prophase, cup-shaped microfilamentous structures appear that are filled with condensing chromatin. Microtubules are also present and are parallel to the division axis.