The mechanisms and dose-response of UV action on the early development of Macrocystis pyrifera (L.) C. Agardh gametophytes were investigated. Post-release, zoospores undergo germination, germ tube elongation, DNA synthesis, nuclear division and translocation, which were followed for 41 h under laboratory conditions. The spores were exposed to UV radiation before germination (3 h post-release) or before nuclear division (20 h post-release). Biologically effective UV-B doses (BED_{DNA300 nm}) higher than those used in the experiments are needed for a 50% inhibition in germination (BED₅₀ > 1600 J m⁻²). Nuclear division/translocation was more sensitive to UV radiation. When the spores were cultured in the dark, UV exposure at both 3 and 20 h post-release resulted in a dose-responsive inhibition of nuclear division/translocation (BED₅₀ 64 and 86 J m⁻²). Culturing in the light indicated recovery in the spores that were irradiated at 3 h post-release (BED₅₀ 356 J m⁻²), whereas no light-dependent recovery occurred within 41 h of culture when irradiated at 20 h post-release (BED₅₀ 80 J m⁻²). The results present a possible mechanism of UV inhibition in early life stages of the giant kelp, suggesting that environmentally relevant UV-B levels can perturb or delay the development and recruitment of the gametophytes by inhibiting nuclear events.