Mutations were induced in tissue-cultured wasabi (Wasabia japonica Matsumura) by treating in vitro-derived shoot tips with either γ-rays or X-rays at 0, 10, 20, 40 or 80 Gy. Doses of up to 40 Gy of either γ- or X-ray treatments resulted in a survival rate of more than 60% in culture after 3 mo. The use of γ- or X-rays at doses between 10 Gy and 40 Gy to induce mutation in W. japonica resulted in an alteration of the growth and allyl isothiocyanate (AITC) content of multiple shoots after 3 mo. in culture on Murashige and Skoog medium containing 5 μM N⁶-benzyladenine (BA). Putative mutants from the 40 Gy treatments of either γ - or X-rays exhibited a reduction in shoot weight, number, and height, whereas treatments of either γ-rays or X-rays at 10 Gy and 20 Gy doses showed no significant differences in shoot growth. All shoots treated with 80 Gy were either necrotic or irregenerable, while those treated with 40 Gy produced deformed leaves, from both types of ionizing radiation. Concentrations of AITC were measured by the use of gas chromatography-mass spectrometry (GC-MS). The accumulation of AITC was shown to decrease when doses increased in both γ- and X-ray treatments, compared with the controls. Positive responses were solely occurred at 18 mo. after transfer of in vitro rooted shoots to the shade house. The survival rate, rhizome weight and AITC content of plants derived from shoots treated with 20 Gy or 40 Gy of either γ-rays or X-rays were significantly greater than those of the controls.