[en] Synchronous V79 Chinese hamster cells were exposed in G₁-phase to either X-rays or 4.6 MeV/u Ar-ions (LET: 1840 keV/μm) and the induction of chromosomal damage was measured at 5 sampling times ranging from 14 to 30 hours after treatment. To distinguish between cells in the first and second postirradiation cycle the Fluorescence-plus-Giemsa technique was applied. The experiment showed that the time course of the appearance of damaged cells is markedly influenced by radiation induced cell cycle delays and depends on both radiation quality and dose. The yield of aberrant metaphases and the number of aberrations per metaphase was found to increase with sampling time, but this increase was more pronounced for Ar ions. These differences in the yield time profiles of X-ray and Ar ion induced chromosomal damage are particularly important for an accurate determination of the RBE for particles. Our data clearly indicate that meaningful RBE values can only be obtained, if chromosomal damage is analyzed at several postirradiation samping times and the complete time course of the expression of chromosomal damage is taken into account. Besides these qualitative differences, differences in the spectrum of chromosomal lesions were observed for X-rays and Ar ions. The experimental results are interpreted on the basis of the different pattern of energy deposition by sparsely and densely ionizing radiation. In addition, a statistical analysis based on the Neyman type A distribution is performed, which takes into account the specific stochastic properties of practile irradiation. (orig.)