We demonstrate the presence of a c-myc protein with an apparent molecular weight of 67 kd in the lower eukaryote Physarum polycephalum with polyclonal and monoclonal anti-c-myc-antibodies. It is shown that the amount of myc protein present in the nucleus does not fluctuate during the naturally synchronous cell cycle of Physarum. The myc protein remains firmly associated with the nuclear matrix after a variety of matrix preparation procedures. Although the level of c-myc protein is invariant during the cell cycle, the matrix-bound portion of c-myc protein is higher during S-phase as compared to G2-period. In immunoelectron microscopy the immunosignal was considerably stronger in the nuclear matrix compared with unfractionated nuclei, indicating increased accessibility of c-myc protein in nuclear scaffold structures. We show that the nuclear localization of c-myc protein changes during the cell cycle, being transiently but specifically bound to the periphery of the nuclear matrix structure during S-phase.