Abstract: The morphology and ultrastructure of the sperm of Cristaria plicata were studied with electron microscope. The results show that the sperm is 40-43μm long and consists of the head,the middle piece and the flagellum. The 1.5×2.6μm sperm head is bullet shaped and contains a nucleus which is condensed and delimited by a nuclear membrane. The middle piece(0.6μm long)composes of five spherical mitochondria. A proximal centriole is locatesd in a fossa in the nucleus base and connects to a distal centriole by a dense amorphous matrix. The distal centriole is located to the flagellar collar. The flagellum is 37-40μm long. A very reduced acrosome is only composed of several acrosomal vesicles.