Genomic instability has been observed in remote progeny of Chinese hamster cells (line V79) exposed to two different accelerated heavy ions – 18O and 20Ne. Linear energy transfer (LET) values of these radiations were in the interval 116-153 keV/µm. Characteristics of the dependence of HPRT mutant fraction on expression time has been measured after exposure to doses: 0.5, 1, and 2 Gy. It was found that mutant fraction is rising with the expression time; reaching maximum; and decreasing again. After approximately 40-45 days (80-95 cell generations), mutant fraction has returned back to the level characteristic for spontaneous mutants, under all irradiation conditions. Position of the mutant fraction maximum was moving with different LET.