Intrachromosomal homologous recombination in Arabidopsis thaliana plants was found to occur in all organs examined from the seed stage to the flowering stage of somatic plant development.  Recombination frequencies were typically in the range of 10  to 10 events per genome.  A. thaliana mutants selected for hypersensitivity to X-irradiation also proved to be simultaneously hypersensitive to the DNA damaging agents mitomycin C and/or methyl methanesulfonate.  The mutants were also deficient in somatic homologous recombination.  These findings suggest that repair of some types of DNA damage requires a recombinational process that was defective in the mutants studied.  In nature, plants are continuously exposed to UV-B (280-320 nm) radiation, a component of sunlight that damages the DNA of somatic cells.  Cyclobutane pyrimidine dimers (CPD) are a type of damage induced by UV-B.  In A. thaliana, homologous recombination appears to be directly involved in repairing CPD damage.