Accumulation and removal of UVBR (ultraviolet-B radiation: 280 to 315 nm)-induced DNA damage (cyclobutane pyrimidine dimers; CPDs) were studied in 2 size fractions (0.2 to 0.8 and 0.8 to 10 mum) of natural populations of tropical marine bacterio- and phytoplankton off Curacao, Netherlands Antilles. Plankton was either directly sampled from the surface layer (mixed situation), or incubated in UVR-transparent bags at 2 water depths (simulated non-mixed situation). A DNA dosimeter was used to measure biologically effective UVBR doses as well as attenuation of biologically effective radiation in the surface layer. A significant difference in CPD level between the 2 plankton size fractions could not be measured. In the mixed as well as in the non-mixed situation, DNA damage was accumulated between 09:00 and 14:00 h irrespective of the variability in incident UVBR. Surface layer samplings on consecutive days showed a clear diel pattern of increase and decrease, indicating that, in the field, DNA damage is repaired or diluted during the afternoon or at night. No significant repair of DNA damage could be measured in samples that were incubated during the afternoon at 0.2 or 10 m depth. The UVBR sensitivity of surface-incubated plankton was expressed as the ratio between damage induced in the cells to damage induced in (unshielded and unrepaired) bare DNA. In both plankton size classes this ratio fluctuated around 0.32, In conclusion, the results show that plankton cells present in the clear waters off Curacao undergo UVBR stress despite the fact that they are subjected to vertical mixing. No significant difference in CPD levels between 2 size fractions of plankton could be measured, suggesting that in this plankton population other aspects, e.g. repair capacities or cell morphology, were more important than cell size.