DNA polymerase β (pol β) cDNA of rat fused to an enhancer-promoter region plus a poly(A) signal sequence of actin 5C gene of Drosophila (abbreviated pol β) was transferred to the Drosophila genome. Three of four constructed transgenic strains possessing transgene pol β on different chromosomes were studied. Levels of the pol β transcript and those of the polymerization activity of pol β were markedly elevated in cultured cells transfected with polβ-bearing vectors as well as in embryos of the transgenic strains. The popular idea that DNA polymerase β participates in DNA repair was not supported by the observation that a pair of a normal and a polβ strain, and the other pair of a mei-9 mei-41 (DNA-repair deficient double mutations) strain and a polβ mei-9 mei-41 strain, showed no difference in survival within each pair after treatment with ultraviolet light, methylmethane sulfonate and mitomycin C. The other idea that DNA polymerase β participates in recombination was supported by the findings that spontaneous frequency of recombination, either meiotic or mitotic, is significantly higher in a transgenic polβ strain than in a non-transgenic strain. The enhanced recombination frequency in the polβ strain may, however, reflect an indirect effect of over-produced pol β proteins on chromosomal stability. Whatever the direct effect of rat pol β is, the transgenic polβ flies will be useful for study of the physiological role of pol β and the mechanism of recombination.