The 129I/127I ratio in marine biological samples and seawater samples collected at the Western Pacific Ocean was measured. There was one order of difference of 129I/127I ratio between the biology and seawater. General isotopic fractionation cannot explain such a large difference. Biology may have preference of iodine chemical forms when they assimilate iodine. We tried to measure the 129I/127I ratio of two major iodine chemical forms (IO3- and I-) in seawater samples and discuss why such the difference of 129I/127I ratio was found between biology and seawater.