A series of characteristic cytological events during fertilization and early embryonic development in Apostichopus japonicus were studied by the fluorescence microscope with HOECHST 33258 stained.The results indicated that unfertilized mature eggs of A. japonicus were globular and remained at the metaphase of meiosis Ⅰ.At water temperature of 22-23 ℃ and salinity 29,sperms quickly attached to the surface of the egg after mixing of sperms and eggs.At 12 min after insemination,the first meiosis is finished,with release of the first polar body.Most of fertilized eggs released the second polar bodies at 20 min after insemination.About 35 min,the male and the female pronuclei associated with each other after their chromosomes formed respectively in the center of egg.The first and second cleavages finished at 80 min and 100 min respectively in fast developing fertilized eggs.Polyspermy phenomena were observed in the fertilization process in A. japonicus.