<p><i>Euglena gracilis</i> is a common phytoplankton species, which also has motile flagellate characteristics. Recent research and development has enabled the industrial use of <i>E. gracilis</i> and selective breeding of this species is expected to further expand its application. However, the production of <i>E. gracilis</i> nuclear mutants is difficult because of the robustness of its genome. To establish an efficient mutation induction procedure for <i>E. gracilis</i>, we employed Fe-ion beam irradiation in the RIKEN RI beam factory. A decrease in the survival rate was observed with the increase in irradiation dose, and the upper limit used for <i>E. gracilis</i> selective breeding was around 50 Gy. For a practical trial of Fe-ion irradiation, we conducted a screening to isolate high-temperature-tolerant mutants. The screening yielded mutants that proliferated faster than the wild-type strain at 32 °C. Our results demonstrate the effectiveness of heavy-ion irradiation on <i>E. gracilis</i> selective breeding.</p> <p>The procedure of screening of mutant <i>Euglena gracilis</i> strain. With this procedure thermal stress resistant strains were segregated.</p
