Abstract: Floraldevelopm ent of Sagittaria guayanensis H.B.K.ssp.lappula(D.Don)Bogin (Alism a- taceae)has been observed under scanning electron m icroscope(SEM).The sepalprim ordia firstinitiates on radiispirally,and a w hirlofthree alternisepalous petalprim ordia form nearly sim ultaneously.Deve- lopm entalstudies confirm thattheexistence of petal-stam en com plex prim ordia isafalse appearanceand do not support that antipetalous stam en pairs are generalcharacteristics offloralconstructure in Alis- m ataceae. Itshows that the first six stam en prim ordia locate at the sites alternate w ith the sepalpri- m ordium and the petalprim ordium,and they are m ore like antisepalous,w hereas antisepalous stam en pairs orantipetalousonesare a secondary nature ofthe alternate arrangem ent.Futhercarpelprim ordium (orrudim entary carpelprim ordium in the m alefloralbud) w ere form ed centripetally,m oreorlessalter- nating w ith the previous ones atsubsequently higher levels.The generaltypes offloralconstructure in Alism ataceae should be revised.Ascom pared w ith the relative genusofAlism ataceae,thecharacteristics offloralconstructure are discussed in relation to the phylogenetic position ofS.guayanensis.