The growth rates and motility of dinoflagellates were studied in the field in the presence or absence of UVB radiation, as well as in the laboratory under artificial radiation conditions. Photosynthetically active radiation (PAR, 400–700 nm) and UVB radiation showed large variations due to cloud cover and seasonal changes in natural daylight. In Swedish coastal water, UVB radiation was attenuated to about 10% of surface irradiance at a depth of 120 cm. There was no significant difference in the motility of two strains of Prorocentrum minimum (Atlantic, LAC4LI; Kattegat, LAC6KA83) kept in the water at different depths (35 and 120 cm) for 4 h, with or without natural solar UV radiation, except for a day with high UVB irradiance (1.2 W m−2), which decreased the motility at a depth of 35 cm for the two species). Simulated in situ experiments with 2 h natural daylight, with and without natural UV radiation (UVB, 1.6 W m−2), had a dramatic effect on the motility of Gyrodinium aureolum. Artificial UVB radiation from UV lamps (4 h, 2.72 kJ m−2 day−1, biologically effective UVB radiation, UVBBE) in the laboratory decreased the motility of Heterocapsa triquetra (LAC20) by 56% and the two strains of P. minimum (Atlantic, LAC4LI; Kattegat, LAC6KA83) by 43% and 36% respectively; the growth was inhibited for all species, as well as for Amphidinium carterae (LAC1KA83), when organisms were exposed to more than 0.7 kJ m−2 day−1 of UVBBE radiation.