The cellular slime mold Dictyostelium discoideum, a lower eukaryote feeding on bacteria in soil, is highly resistant to DNA damaging agents. D. discoideum expresses at least two RecA homologs, Rad51 and mitochondria localizing RecA. Rad51 consisting of 351 amino acid residues is 69% identical to human Rad51. UV light-irradiation and H2O2-treatment increased the expression level of Rad51, suggesting that Rad51 is required for DNA repair in D. discoideum. A gene disruption mutant was not viable, suggesting that Rad51 is essential for D. discoideum.