The ultrastructure of interphase and mitotic nuclei of the epimastigote form of Trypanosoma cyclops Weinman is described. In the interphase nucleus the nucleolus is located centrally while at the periphery of the nucleus condensed chromatin is in contact with the nuclear envelope. The nucleolus fragments at the onset of mitosis, but granular material of presumptive nucleolar origin is often recognizable in the mitotic nucleus. Peripheral chromatin is in contact with the nuclear envelope throughout mitosis, and it seems reasonable to assume that the nuclear envelope is involved in its segregation to the daughter nuclei. Spindle microtubules extend between the poles of the dividing nucleus and terminate close to the nuclear envelope. The basal body and kinetoplast divide before the onset of mitosis and do not appear to have any morphologic involvement in that process. Spindle pole bodies, kinetochores, and chromosomal microtubules have not been observed.