Mitotis in the choanoflagellate Monosiga sp. has been studied by electron microscopy for the first time based on serial sections of metaphase and anaphase. The dense metaphase plate is formed by chromosomes at the centre of the nucleus. Spindle poles are located at the opposite sides of this plate. The nuclear envelope is intact except for fenestrae at the poles through which the spindle microtubules from the MTOCs penetrate. The MTOC is formed by the entire flagellar apparatus of the interphase cell, often with an axoneme lying in cytoplasm. At anaphase, the chromosomes move to the poles with different speeds and the nuclear envelope fragments and disappears.