The free-living soil nematode, Caenorhabditis elegans has been developed as a model for evaluating the effects of ionizing radiation. The highly reproducible developmental program and simple anatomy of this species have been exploited for the variety of in vivo cellular targets useful for radiobiological measurements. The ease of genetic manipulation and relatively simple genome complement the anatomical features to facilitate measurements of genomic damage and repair.Nematodes have been tested in a number of assays for the production of mutations in large genetic regions and specific genes, aberrations in germ cells and somatic cells, disruption of developmental programs and cell survival. A systematic survey of different radiation types reveals that the frequency of radiogenic endpoints varies with dose and exhibits a strong dependence on the radiation quality as measured by the linear energy transfer (LET). Specific dependence on velocity and charge are also implicated in these measurements.Radiation species investigated include UV light, 60Co gamma rays, fission spectrum neutrons and charged particles from atomic number 1 to 57. Specific energy ranges explored for particles are from 150 to1060 MeV/amu. Two space flight experiments extended these measurements with worms to naturally occurring fields of radiation in low earth orbit.Together these experiments represent one of the most comprehensive surveys of high LET radiation effects in a single animal test system For purposes of risk assessment the frequency versus dose (or fiuence) relationships have been converted to cross sections or probability per particle so as to normalize between endpoints and extrapolate to other test systems.