Abstract: A network was shown in the nucleus of special dinoflagellate,Oxyrrhis marina,by serial selective extrations,diethylene glycol distearate (DGD) embedment sectioning and embedment--free electron microscopy.This reticular structure could resist against the extractions of CSK buffer and the buffer which always removes microtubules and microfilaments from the cells.It could also resist against the digestion of Dnase and the treatment of hot trichloroacetic acid (5%,90-91℃,15 min).Therefore,it must be a non-DNA fibrous protein structure.Its reticular form,the thickness of the filaments (2.8-24 nm),connection with intermediate filaments in cytoplasm and the containing of a small amount of RNA which is necessary in keeping its spatial organization etc,are very similar to those of the nucleoskeleton of typical eukaryotes.However,the lamina of Oxyrrhis marina is not a well-distributed layer as the lamina in typical eukaryotic cells,but a discontinuous imcomplete layer.The nucleoskeleton was first demonstrated in the nucleus of a special dinoflagellate,Oxyrrhis marina.