Purpose: To assess the ability of the Auger-emitting nuclide, zinc-65 (65Zn), relative to γ-irradiation, to cause chromosomal aberrations in cultured rat prostate cells. Materials and methods: Rat prostate adenocarcinoma cells in culture were exposed to doses of 1, 2, 3 or 5 Gy of external γ-irradiation for 24 h or incubated with 0.7, 1.5, 1.8 or 2.8 MBq of 65Zn for 24 h. The uptake by and clearance from cells of 65Zn was measured. Metaphase spreads prepared from washed cells were scored for chromatid- and chromosome-type aberrations. Results: Following exposure to 65Zn or γ-irradiation, chromatidtype damage was more commonly observed than chromosometype aberrations. The relationship between induced chromatid damage and gamma dose (to 3 Gy) was best fitted by a second-order polynomial function, while the activity-response relationship for chromatid damage caused by 65Zn appeared to be best fitted by a straight line. Measurements of the uptake of 65Zn by cells showed that average concentrations within cells were about 100 times the concentration in the culture medium. Assuming uniform distribution of 65Zn within cells, with 36% in the nucleus, the dose was estimated as 0.70 Gy per MBq added 65Zn, with Auger electrons contributing most (93%) of the dose. Assuming that 20% of cellular zinc was localized in the nucleus, based on previous measurements, the dose to the nucleus was calculated as 0.44 Gy per MBq added 65Zn. RBE values for chromatid damage induced by 65Zn compared to γ-radiation range from about 1 to 3 based on a uniform dose throughout the cell and from about 2 to 5 based on 20% of 65Zn in the cell nucleus. Conclusion: The observed radiotoxicity of 65Zn is consistent with its behaviour as an Auger-emitting radionuclide that is localized to some extent in the nucleus.