Summary—The heliozoanDimorpha mutanspossesses a complex centrosome, consisting of an axoplast which anchors radiating axopodia crossing the nucleus, two kinetosomes and different associated fibres, including striated roots. During mitosis, which is an open orthomitosis, the axoplast separates into two, then dissociates, and most of the axopodia disappear. The axoplast therefore does not transform directly into asters at the poles of the mitotic spindle, as previously suggested. The spindle poles are made up of diffuse material which appears to be linked with a pair of kinetosomes by a short striated fibre. No differentiated kinetochore was observed. At the end of telophase, some axopodial axonemes appear to be reconstituted by the juxtaposition of microtubules before the axoplast reappears. The channels followed by the axopodial axonemes crossing the nucleus were formed when the nuclear envelope reconstituted around the chromosomes. From these observations, and those on other protists, it is suggested that the centrosome results from the concentration during the cell cycle of a heterogenous group of microtubule organizing centers (MTOCs), of which the centriole might be only one of the type