[en] Distributions of radionuclides (⁶⁰Co, ¹³⁷Cs, ⁹⁵Zr-⁹⁵Nb and ¹⁰⁶Ru-¹⁰⁶Rh) among green alga (Ulva pertusa), sea water and marine sediment were examined by radioisotope tracer experiment in order to estimate the influence of sediment on the accumulation of radionuclides by the alga. By the application of the compartment model to the experimental results, exponential formulas of distributions were obtained. Through comparison of the transfer coefficients of radionuclides calculated from the exponential formulas, the influence of the sediment on the accumulation of the radionuclides by the green alga was determined to be the largest for ⁶⁰Co, followed by ⁹⁵Zr,-⁹⁵Nb, ¹⁰⁶Ru-¹⁰⁶Rh and ¹³⁷Cs in this order. The activity ratios of ⁹⁵Zr-⁹⁵Nb and ¹⁰⁶Ru-¹⁰⁶Rh calculated from the transfer coefficients are larger for the alga than for the sediment, inversely those of ⁶⁰Co and ¹³⁷Cs show higher values for the sediment than for the alga. Especially, in the case of ⁶⁰Co, the activity ratio for the sediment is approximately 20 times greater than that for the alga. Biological half lives in green alga estimated from the transfer coefficients were 10 days for ⁶⁰Co, 7 days for ¹³⁷Cs, 26 days for ⁹⁵Zr-⁹⁵Nb and 24 days for ⁹⁵Zr-⁹⁵Nb and 24 days for ¹⁰⁶Ru-¹⁰⁶Rh. (auth.)