Photomovement measurements were carried out with swarmers of the brown algae Scytosiphon lomentaria (Lyngb.) Link and Petalonia fascia (O. F. Müll.) as a function of irradiance direction, photon irradiance, spectral composition and ultraviolet radiation (UVR, lambda=280-400 nm) dose. Swarmers from both species showed similar photomovement patterns: negative phototaxis occurred under photon irradiances of 10-90 micromol photons m(-2) s(-1), and no movement was observed at 190 micromol photons m(-2) s(-1). The translocational velocity measured between 10 and 90 micromol m(-2) s(-1) ranged from 100 to 200 microm s(-1). The accumulation of swarmers presented a peak at 450 nm (waveband of 50 nm), and smaller peaks at 400 and 500 nm; no effect was observed at wavelengths of 550 nm and above. The decline in phototactic index (an estimator of photomovement response) of swarmers was linearly correlated with the logarithm of UVR doses. These data were correlated with levels of natural solar radiation in the field. It is hypothesized that motility of swarmers could be a critical factor in the survival of these species under a scenario of increased UVR.