We investigated the effect of ultraviolet‐B radiation (280–320 nm) on the growth and motility of the marine dinoflagellate Gyrodinium aureolum in the presence of humic substances. The specific growth rate after 7 d of the UV‐BBE (biologically effective radiation) exposure (2.15 kJ m−2 d−1) decreased by 92% compared to the control. The growth rate for UV‐B treated cells increased by 18, 26, and 10% when cells were cultured in medium enriched with humic substances (1.7, 4.2, and 8.3 mg liter−1). However, the growth rate was lower at the highest humic substance than at the two lower concentrations. When humic substances were used as a filter to shade the organisms from UV‐B radiation without substantial changes in the amount of white light, instead of being added to the medium, the growth rate increased with the concentration of humic substances. UV‐B radiation had little or no effect on the motility of G. aureolum, whether humic substances were added or not. Phototactic orientation of the UV‐B‐treated cells was significantly weaker than the control to actinic white light (400–700 nm, 47 m−2). The presence of humic substances during treatment with UV‐B radiation reduced the inhibition of the positive phototaxis in G. aureolum.