To reveal the in planta behaviour of caesium (Cs), the stable isotope 133 Cs was administered into 3-year-old Cryptomeria japonica seedlings by the application of 133 CsCl aqueous solution to the bark surface. The administered 133 Cs was quantified by ICP-MS measurements, which showed transportation of 133 Cs in an ascending direction in the stem. Distribution of 133 Cs was visualized using freeze-fixed C. japonica woody stem samples and cryo-time-of-flight secondary ion mass spectrometry/scanning electron microscopy (cryo-TOF-SIMS/SEM) analysis. Cryo-TOF-SIMS/SEM visualization suggested that 133 Cs was rapidly transported radially by ray parenchyma cells followed by axial transportation by pith and axial parenchyma cells. Adsorption experiments using powdered C. japonica wood samples and X-ray absorption fine structure (XAFS) analysis suggested that 133 Cs was in the hydrated state following its deposition into tracheid cell walls. [ABSTRACT FROM AUTHOR]