Spores and germlings of six red algal species were cultivated for 15 weeks in coloured light fields at low intensities. The photon fluence rates at which growth exceeded growth in dark-kept controls were, in green light, 0·1 μmol m^-2s^-1 in Audouinella daviesii, 0.5 μmol m^-2s^-1 in Halarachnion ligulatum, Pterothamnion plumula, Chondrus crispus and Plumaria elegans, and 1·0 μmol m^-2s^-1 in Ceramium rubrum. In blue or red light, at least 1·0 μmol m^-2s^-1 was required for germling growth in these species, except for P. elegans which required 0·5 μmol m^-2s^-1 in blue light. The advantage of green light for growth was particularly evident in A. daviesii which formed no more than two cells in darkness and in photon fluence rates up to 1 μmol m^-2s^-1 of blue or red, but one further cell after 15 weeks at 0·1 μmol m^-2s^-1 in green. Germling growth of the green alga Ulva pseudocurvata required at least 3 μmol m^-2s^-1 in green, but only 1·0 μmol m^-2s^-1 in blue or red light. White fluorescent light levels permitting growth in red algae were higher than those in green light and equal to minimum requirements in blue or red light in the majority of species investigated. Settled spores of H. ligulatum and the green alga Chaetomorpha melagonium survived for at least 1 year in darkness, whereas spores of C. crispus survived only 39 weeks and those of 11 other red algae and U. pseudocurvata survived 26 weeks or less. Light saturation and light inhibition of growth occurred at higher photon fluence rates in the lower eulittoral species Ceramium rubrum, C. crispus and U. pseudocurvata (saturation, 200-300 μmol m^-2s^-1; inhibition, 300-500 μmol m^-2s^-1) than in the strictly sublittoral species H. ligulatum and P. plumula (saturation, 10-20 μmol m^-2s^-1; inhibition, 50-100 μmol m^-2s^-1).