Seven species representative of the benthic diatom community of the tidal flats in the Dutch Wadden Sea hardly differed in their sensitivity to ultraviolet-B radiation (UVBR). Some isolates had been cultured in the laboratory for up to 20 yr. Cell numbers of all species increased at a rate similar to unexposed cultures up to a DNA-weighted daily UVBR dose of 3.5 kJ m(-2) d(-1) (biologically effective dose, normalized at 300 nm); only at higher UVB irradiance levels did the growth rate become reduced. No clear relationship between mean cell size and UVBR sensitivity was observed. The benthic diatoms that were tested are apparently adapted to the natural, high UVB irradiance incident at tidal flats during spring and summer. Thus, even a sharp UVBR increase resulting from severe stratospheric ozone reduction would hardly affect tidal flat diatom communities by influencing cell division rate. In contrast, growth of representatives of the phytoplankton community was already seriously affected by doses that were 10 times lower. This is in agreement with their natural, low UVBR exposure.