Abstract: The surface structure of the antennule of Macrobrachium rosenbergii ,an antennule type with and appendage flagellum,were observed in detail with stereomicroscope and scanning electron microscope.The results revealed that:1)the antennule of M.rosenbergii was composed of inner flagellum or medial flagellum,outer flagellum or lateral flagellum,and three scapes.The outer flagellum consisted of an appendage flagellum and a main flagellum(Plate Ⅰ∶1);2)there were above 250 plumose setae on the whole scape portion(Plate Ⅰ∶2),in addition,there were several hard thorn setae(Plate Ⅰ∶4)and some bamboo shoot-like setae(Plate Ⅰ∶3)whose function is not clear at present;3)the inner and main flagella were extremely long(Tab.1)and their annuli were well-marked.There were 4～40 soft thorn-like setae on each segment of the flagella,whose top sealed and were thin.The shape of the born socket of the setae was U-type(Plate Ⅰ∶5);4)there was a concave slot on the ventral side of each appendage flagellum(Plate Ⅰ∶5,Plate Ⅱ:7,10,11).In the slot,there existed above 200 aesthetascs that were similar to the ones of Penaeus vannamei in quantity(Plate Ⅰ∶5,Plate Ⅱ:7—2)16 ,but obviously fewer than that of the most of other decapod species known 4,8,12,15 ;5)the slot of appendage flagellum wrapped up in longitudinal direction to display a cylindrical status(Plate Ⅱ∶10),which maybe had a defensive function for their aesthetascs;6)each aesthetasc was divided into two parts,one was a hard tube part(lower part),the other was a membrane tube part(upper part)(Plate Ⅱ∶8).There were several segments along the hard tube part,and the wall of the part was thick.However,wall of the membrane tube part was extremely thin which maybe makes the chemical substances entry easier(Plate Ⅱ∶9);7)many bud-shape setae emerged from proximal segments of the slot of the appendage flagellum,and these setae were some primary aesthetascs as they became gradually to aesthetascs in whose appearance were completed(Plate Ⅱ∶11,12). In addition,we suggested that the antennules of several decapod species known should be classified according to basic appearances of their antennules(Tab.2).They were Crab-type8,9,12,15,20,Lobster-type3—5,10,11 ,Crayfish-type17—19 and Prawn-type13,14,16.However,since the antennule of M.rosenbergii possess a particular appendage flagellum as well as its configurations,the antennule was geared to a new type, “Macrobrachium rosenbergii -type".Moreover,according to the characteristics of configurations of the appendage flagellum, M.rosenbergii was respectively deferent from P.argus4,10,11,L.vannamei16,andS.serrata15in defence structure model for their aesthetacs(Tab.3).