Conidia, derived from a strain of Aspegillus nidulans known to carry a specific chromosomal duplication, were irradiated. The duplicated segment had genetic markers, which, when eliminated form the genome, allowed the easy detection of deletion mutants. Survival curves derived following 15 MeV electron and γ-ray irradiation wre characterized by the presence of an appreciable shoulder, whilst 50 kvp X-rays gave a much smaller shoulder. Irradiation with β-particles and α-particles gave rise to exponential survival curves. The RBE values for these radiations based on the D 37 value were for γ-rays, 1.0, 15 MeV electrons 1.0, 50 kvp X-rays 1.9, β-particles 2.1 and α-particles 2.1 and α-particles 3.4. With the exception of γ-rays the radiations described were compared with respect to their ability to induce chromosomal deletions. When the number of deletants amongst survivors was plotted against dose, a linear relationship was found for electrons, X-rays and β-particles. The response recorded for α-particles was essentially linear but with a biphasic component. The RBE values for the radiations, based on a value of unity for 15 MeV electrons were as follows: X-rays 1.3, β-particles 0.8, α-particles above 7.5 krad 2.3 and below 7.5 krad 3.5. When these same data were re-plotted with number of deletants amongst survivors against log survival, electrons appeared the most efficient radiation at producing deletants amongst survivors, with an “m value” of 283 × 10 −5. Tritiated water was least efficient, the corresponding value being 182 × 10 −5. The number of deletants per 10 4 conidia plated, when plotted against dose yielded a curve which increased to a peak and then decreased linearly for all radiations. The peaks for electrons, X-rays and α-particles each had a value of about 14 deletants per 10 4 conidia plated and the peaks roughly corresponded with the point at which the survival curve became exponential and was clearly indicative of the accululation of sub-lethal damage. However, for β-particles the peak had a value of 7 deletants per 10 4 conidia plated. A non-DNA target has been implicated for cellular death following β-particle irradiation.