[en] The recent accumulation of the long-lived isotope of iodine, ₁₂₉I, which is released in environment by the peaceful use of nuclear energy or nuclear test explosion is becoming important in the view point of the internal exposure by the low level radiation. The studies on the detection of determination of ₁₂₉I in environmental samples so far published are still very few. The authors tried to detect ₁₂₉I in some Japanese seaweeds and soil samples with the aid of the activation method by using the nuclear reaction of ₁₂₉I(n, ß)₁₃₀I. The samples analysed in this work are tangle (Laminaria japonica) for daily food grown in Hidaka, Hokkaido and uncultivated soil collected in Tokai, Ibaraki Pref. As the ß-ray peak indicator for ₁₃₀I, cesium oxide and the aged radioisotope product of ₁₃₁I are also subjected to the neutron irradiation. From cesium oxide, ₁₃₀I is formed by the reaction of ₁₃₃Cs(n, α)₁₃₀I. An aged vial of the ₁₃₁I product is expected to contain very minute amounts of ₁₂₉I which is also produced both by the fission of uranium and neutron capture reaction of tellurium followed by ß^--decay. The ß-ray spectra for the soil sample, cesium oxide and the aged ₁₃₁I vial are shown in Fig. 1. No appreciable peak was found for the seaweeds sample. In the ß-ray spectra for irradiated cesium oxide and the aged ₁₃₁I vial, several typical peaks for ₁₃₀I were observed. By comparing with these peaks, several small peaks which appear at around 418, 536 and 739 keV in the soil sample can be attributed to those of ₁₃₀I. The ₁₂₉I content in the soil sample is roughly estimated to be 2 x 10^-₁₀ Bq/g. (author)