We report here a bystander effect in chromosomal damage using He-ion microbeam. Human-hamster hybrid cells were irradiated with a precision He-ion microbeam generated by the Columbia microbeam system. When 20% of the cells were exposed to single He ion, the incidence of cells with chromatid-type breaks detected with the PCC technique was covered wide range from 0 to 6 breaks per cell. In contrast, the distribution showed a mixed two-peak pattern, such as non-exposed and all-cell exposed patterns, under the condition of assuming no bystander effect by treating with an effective inhibitor of cell-cell communication. These findings provide clear evidence that single He-ion irradiated cells can induce bystander chromosomal alterations in neighboring cells not directly hit by He ion.