Abstract: There are evidences showing that diplomonads would be the most primitive eukaryotes among all present-existing organisms.Therefore,the informations about diplomonads nucleus are very important for investigating the origin and early evolution of cell nucleus.However,they are very seldom in literature.The nucleus of Giardia lamblia was investigated as a representative of diplomonad nucleus with electron-microscopy.The perinuclear space within the nuclear envelope of Giardia is very prominent and on this background we found that there were openings on the nuclear envelope,which were very much larger than the nuclear pores.The sixe of the opening varied in different nuclei.At the margin of the opening.the nuclear inner membrane is continuous with the nuclear outer membrane.Therefore,the opening can not be the artefact due to sectioning.Usually,at the opening,there is a nuclear lamina-like thin layer separating nucleoplasm from cytoplasm.The layer seems not membranous,because when Giardia cells were fixed with KMnO4 to preserve only membranous structures,this layer were absent.The layer might perhaps be a part of the nuclear lamina of Giardia.Although after a series of specific extractions to demonstrate Giardia nuclear matrix;nuclear lamina could not be found (Dai et al.,1996),it might still be exist but only so primitive that could not bear the routine extrations.Nuclear envelope must have originated from the primitive endoplasmic reticulum in the closest prokaryotic ancestor of eukaryotic cells (Li,1979).Those parts of the primitive endoplasmic reticulum,which were directly neighboring to the"nuclear region"of the prokaryotic ancestor evolved into nuclear envelope (Li,1996).According to this hypothesis,at the earlier stage of this evolutionary process,the newly formed primitive nuclear envelope could not enclose the whole nuclear region and there must have remained many openings.The discovery of nuclear envelope openings in Giardia nucleus has confirmed this deduction.