Abstract: Our laboratory and Shanghai Institute of Cell Biology have done a series of works on the cell biology and biochemistry of special dimoflagellate,Oxyrrhis marina for many years.The present work is one amony them.The fine features of the constantly condensed chromosomes of the special dinoflagellate Oxyrrhis marina and the effects of different fixations on the fine featrues were observed and compared with those of the nucleoid of a species of human excrement bacteria,and of the constantly condensed chromosomes of a typical dinoflagellate Prorocentrum micans and a flagellate Euglena sp..Materials were fixed with glutaraldehyde (3%,pH7.0),OsO4(1.5%,pH7.0),and glutaraldehyde-OsO4,respectively,The bacteria were also fixed with Ryter and Kellenberger s so-called "standard method".Ultrathin sections were made and stained with uranyl salt routinedly.The fine features of the bacterial nucleoid varied greatly depending upon the fixation methodes used.On the contrary,the features of the condensed chromosomes of Euglena sp.showed no evident changes when different fixation methods were used.As for the condensed chromosomes of typical dinoflagellate,P.micans,both OsO4 fixation and glutaraldehyde-OsO4 fixation gave the typical banding features of dinoflagllate chromosomes,while glutaraldehyde fixation resulted in quite different features.Being quite different from the condensed chromosomes of common dinoflagellates,the condensed chromosomes of the special dinoflagellate,Oxyrrhis marina,showed fine features somewhat similar to those of the Euglena chromosomes Different fixation methods did not give evident changes on the fine features of Oxyrrhis chromosomes either.In this respect,O.marina chromosomes were similar to the condensed chromosomes of Euglena and distinct from those of other dinoflagellates.Lots of evidences in cell biology and molecular biology,that have been accumulated,indicated that Oxyrrhis is quite different from other dinoflagellates,but similar and closer to other flagellates.Oxyrrhis chromosomes are not dissolved when DNA:is removed; Oxyrrhis DNA does not contain hydroxymethyl uracil;Oxyrrhis chromatin contains histone-related protein and nucleosome-like particles;its nucleus divides by means of a special endomitosis,which is very different from the dinomitosis of typical dinoflagellates.The present work provides the differece in a new respect.All these facts indicate that Oxyrrhis represents a special group of flagelles which link the common flagellates with dinoflagellates.A new phylum,Phylum Oxyrrhinea,is suggested.This idea tallies with the result of the molecular evolution study of 24S rRNAs of Oxyrrhis and verious dinoflagellates (Lenares et al.,1991).