Two strains of 6–8-y old Actinia equina (Anthozoa: Actiniaria) asexually derived from two individuals collected on the coast of Brittany were artificially UV irradiated in long-term experiments in the laboratory. After the animals had been allowed to adjust to the test aquaria for 15 months, a programme of UV irradiation and multiparameter analysis continued for 24 months. Ultraviolet irradiated sea anemones obtained in addition a quasi-solar UV irradiation at intensities representing the daily and annual cycles. Of the three modes of UV irradiation tested (1X, 2X and 4X), the one with the lowest intensity and duration (1X) corresponded approximately to the irradiation to which sea anemones are exposed in their natural habitat in the tidal zone of the southern North Sea. In all three modes all the actinians showed a similar, highly significant quasi-exponential weight loss immediately after the onset of UV irradiation. The more the animals were irradiated, the later juveniles were released from the gastrovascular cavity. At 2X the number of active animals gradually decreased after 18 months, whereas with the 4X irradiation mode the amount of activity fell abruptly and significantly after only eight months and remained at a very low level. Detachment from the substrate was also induced by UV irradiation. The experiments showed genotype-specific differences in body mass and migration. The crucial findings were that A. equina individuals that had adjusted to the laboratory for a long time did give clear responses to UV doses and intensities corresponding approximately to the irradiation in their natural intertidal environment or by radiation increased severalfold. The responses were immediate and sometimes irreversible to UV even in naturally occurring intensities and doses. The genotype specificity of some responses provides evidence that the stressor UV could induce genetic modifications of the population.