Somatic pink mutation frequencies in the stamen hairs of Tradescantia BNL 02 clone grown for 76 days in two soil samples taken from the Bikini Island (where a hydrogen bomb explosion test had been conducted in 1954) were investigated. A significantly high mutation frequency (2.58±0.17 pink mutant events per 10³ hairs or 1.34±0.09 pink mutant events per 10⁴ hair-cell divisions) was observed for the plant grown in one of the two Bikini soil samples, as compared to the control plants (1.70±0.14 or 0.88±0.07, respectively) grown in the field soil of Saitama University. The soil sample which caused the significant increase in mutation frequency contained 6,880±330 mBq/g ¹³⁷Cs, 62.5±4.4 mBq/g ⁶⁰Co, and some other nuclides; a 150 μR/hr exposure rate being measured on the surface of the soil sample. The effective cumulative external exposures measured for the inflorescences of the plant grown in this soil sample averaged at most 60.8 mR, being too small to explain the significant elevation in mutation frequency observed. On the other hand, internal exposure due to uptake of radioactive nuclides was estimated to be 125 mrad (1.25 mGy) as an accumulated effective dose, mainly based on a gamma-spectrometrical analysis. However, it seemed highly likely that this value of internal exposure was a considerable underestimate, and the internal exposure was considered to be more significant than the external exposure.