Growth rates in terms of area expansion per 30 min were measured in the red alga Porphyra leucosticta under different light/dark (LD) cycles, irradiance levels and light qualities. Twelve hours per day under white light (35 μmol m−2 s−1) promoted a dramatic and rapid increase in thallus expansion during the first part of the light period followed by low values during the remaining light and dark phase. Light/dark regimes of 8[ratio ]8 h, 6[ratio ]6 h, 3[ratio ]3 h and 1·5[ratio ]1·5 h resulted in a discontinuous growth pattern where thallus expansion was restricted to the light phase while no growth occurred in the dark phase, and there was even a significant thallus contraction just after light-off. Growth rates in terms of percentage area increased with irradiance at LD 12[ratio ]12. This was due to: (1) the phase of high growth rate following the initial ‘morning peak’ extending from the first 6 h of the light phase to cover almost the entire light phase, and (2) an increase in growth rate during the second half of the night phase. Saturation was reached at an irradiance of 125 μmol m−2 s−1. An irradiance of 500 μmol m−2 s−1 decreased thallus expansion rate during the light and dark culture phases relative to the other irradiances used. Light quality also caused changes in growth rate: the maximum value was reached under yellow light followed by red; both of these were higher than the growth rate under white light. Blue light (35 μmol m−2 s−1) caused a decrease of more than 50% in thallus expansion compared with white or red light due to a very dramatic decrease in the ‘morning peak’ of thallus expansion and very low values in the rest of the light and dark phases. Increasing the percentage of blue light in different white light sources also decreased thallus expansion. It is suggested that light influences the growth of Porphyra in three ways: as an external signal matching thallus expansion with light and dark cycles; as an energy source promoting thallus expansion with increasing irradiance; and, at a morphogenetic level, inhibiting growth in blue light.