Macromorphological and micromorphological characters of fruits in the genus Damasonium (Alismataceae), comprising five species, were examined by stereoscope and scanning electron microscope(SEM). We found that the fruits of this genus are usually follicles with 2～6 seeds, rarely achenes with a single seed, compressed, irregularly rectangular with long-beaks at apex, or triangulate with indistinct beaks, carpodermis covered by cuticle
which is again covered densely with fragmentary or granulate waxy appendage. These characters, in authors’ opinions, are of some taxonomic significance in this genus. Combined with the other characters of this genus, we consider that the treatment of D. alisma and D. stellatum as two separate species is reasonable; D. californicum is still a member in the genus Damasonium, and D. polyspermum with some special characters is different from D. alisma. A key to the species of Damasonium is given in this paper. Stomata on the carpodermis in Damasonium, which are unique in the Alismataceae, are first reported.