Abstract Synchronized and oriented egg cells of L. stagnalis have been irradiated with UV light during the G2 phase of the 2nd cleavage cycle; next the egg cells were treated with caffeïne (3 × 10 −3M) for varying periods during the 2nd and the 3rd cleavage cycle. In a second experiment the egg cells were irradiated at the same stage in the G2-phase with different UV doses, and then treated with caffeïne until the beginning of the 3rd cleavage. In the caffeïne-treated egg cells the two cleavage cycles following irradiation appeared to be lengthened, whereas in non-caffeïne controls only the first cleavage cycle after irradiation is longer. It is concluded that the extension of the 3rd cleavage cycle is to be attributed to a prolongation of the S phase, which is not caused by an inhibition by caffeïne of the initiation of the DNA synthesis. The results point to an effect of caffeïne on the post-replication mode of repair; lesions in the DNA are not rendered innocuous in the presence of caffeïne. At the beginning of the next cleavage cycle these lesions still have an inhibiting effect on the progression of DNA synthesis. Morphogenesis was reduced by increasing UV doses; with caffeïne less than 7% of the embryos developed normally. These results may indicate an error-prone repair mechanism.