Abstract: Morphology and ultrastructure of mature sperm of Thenus orientalis were studied with electron microscopy.Each sperm comprises a nucleus,membrane complex area and acrosomal region.The nucleus limited by a nuclear envelope contains uncondensed chromatin,microtubules and filaments.Five to six radial arms extend from the nucleus,and are stationary as well as packed with microtubules.The membrane complex area,which lies between the nucleus and the acrosomal region,contains numerous lamellar membrane structure and vesicle which originate from the lamellar membrane structure.The acrosomal region includes acrosomal vesicle and periacrosomal material.The acrosome is structurally complex and comprised of acrosomal cap,inner and outer acrosomal material.The periacrosomal material is finely granular with most of it lying at the sides of the acrosomal vesicle.The mode of acrosome reaction of Palinura sperm was also discussed in this paper.