Mutation induction in the HPRT gene of the V79 Chinese hamster cells has been observed after exposure to three types of accelerated ions: O-18, Ne-20, and B-11. Four different irradiation conditions were used with the linear energy transfer (LET) values in the range of 50 − 153 keV/µm. The dependence of the mutant fraction on expression time (the incubation period of the cells from irradiation until reseeding on a selective medium containing 6­thioguanine) was measured for the doses of 0.5, 1, and 2 Gy. The dependance non-linear progression was observed under all irradiation conditions. The mutant fraction was increasing with expression time; reaching maximum; and falling back to the initial levels. It was found that after approximately 35 – 45 days (70 – 90 cell generations) the mutant fraction returned to the levels typical for spontaneous mutants. The mutant fraction reached its maximum in different expression times under various irradiation conditions. The position of this peak was moving with the LET value of the used radiation. With increasing LET, the maximum shifted towards longer expression times. For example, the mutant fraction maximum was observed 7­14 days after O-18 irradiation with LET ~ 116 keV/µm; however, Ne-20 irradiation with LET ~ 153 keV/µm caused the peak to shift to 20 – 26 days after exposure. The changes in the mutant fraction values were also compared to the changes in the number (and quality) of chromosome aberrations in the same time periods.