The ultrastructure of spermatozoon and the cytological change of fertilization process in Meretrix lamarkii were systematically studied by electron microscope and fluorescence microscope.The results indicated that the mature spermatozoon(45.2-47.7 μm in total length)of M.lamarkii, belonging to the primitive type,consists of head,middle piece and tail.The head,like a long cocoon in shape(2.5 μm in length),is composed of acrosome and nucleus.The acrosome is conical in shape and has a subacrosomal space inside.Nucleuses are cylindrical with finegrained dense chromatin,having front nuclear pocket and posterior nuclear pocket.The centriolar complex,including proximal and distal centrioles,and surrounding 5 spherical mitochondrias,make up the short middle piece.The tail,a whiplike flagellum,consists of axoneme that is a typical “9+2” microtubular structure and wrapped by an wavy plasma membrane.The cytological change of nuclear behavior during fertilization and early cleavage was observed under the fluorescence microscope.Like most of marine mollusks,the unfertilized mature eggs of M.lamarkii were globular and remained at the metaphase of the first maturation division.At water temperature of 27-28 ℃,artificial insemination was conducted.6 min after fertilization,sperm has penetrated into cytoplasm of egg and activated the maturation division.The fertilized eggs released the first polar body about 12-15 min and the second polar body about 20-25 min after fertilization.About 30 min,sperm nucleus and the haploid female nucleus developed into the male and female pronuclei.35 min,the male and female pronuclei matched into an association nucleus after their chromosomes formed respectively in the center of egg.40-45 min,the chromosomes were separated equally into two daughter cells which are different obviously in size.55-60 min,the second cleavage finished and formed four daughter cells,one big and three small.The process of the second cleavage was fundamentally similar to the first cleavage.75-80 min,the third cleavage which belongs to the type of spiral holoblastid cleavage was completed.In addition,the polyspermy with an incidence of about 1% in the process of fertilization was also observed.Though the polyspermy didn’t affect the formation of male pronuclei,it significantly affected the process of the maturation division and the first cleavage as a result of abnormal chromosome division.