The samples of Sipunculus nudus were treated with 0.04% colchicine solution for 12 h.The celomic liquid was extracted,treated with 6.5‰ seawater for 45 min,and then centrifuged at 1 500 r/min for 8 min.The sediment was fixed by Carnoy's solution.The suspension was dropped on heated slides.The slides were stained with Giemsa(5%,pH 6.8),rinsed in distilled water and air-dried.Well-spread metaphase chromosomes were examined and photographs were taken with a photomicroscope.The chromosome types were categorized according to Levan's report.It was found that the basic number of chromosomes of S.nudus was x=17,2n=2x=34 and the karyotype formula was 2n=2x=26m+8sm,NF=68.All chromosome pairs were metacentric,except that the chromosome pair numbers 2,3,4,8 were submetacentric.Arm ratio was in range of 1.211-2.617.No sex chromosomes and satellites were observed in the present study.