Optimum culture conditions for conchocelis growth and conchospore production were determined for the winter annual Porphyra linearis (Rhodophyta, Bangiaceae) from western Ireland. Conchocelis cultures were established on scallop shells under a range of temperatures (10degreesC, 13degreesC, 15degreesC and 20degreesC), photon irradiances (5, 10, 20 and 40 mumol photons m(-2) s(-1)) and daylengths (8: 16, 11 : 13 12: 12, 13: 11 and 16: 8 h light-dark). Both conchocelis area (mm(2)) and conchosporangial density (number of conchosporangial colonies per mm(2) of conchocelis area) were assessed using an interfaced digital image processor. Under all conditions, conchocelis area increased up to 40 d, but thereafter no further increase was observed. The number of conchosporangial colonies peaked after 40 d for all treatments and then decreased. This reduction in density coincided with conchospore release. Conchosporangial density was highest at 20degreesC, intermediate at 15degreesC and 13degreesC and lowest at 10degreesC. At all temperatures, conchosporangial density was greatest in treatments established under long-day conditions. There was no difference in conchosporangial growth between treatments. The transfer of cultures grown at different daylengths for 30 d to daylengths of 16: 8 h light-dark did not stimulate any increase in conchosporangial density or conchospore release at any temperature. The results indicated clearly that the optimal conditions for conchocelis growth and conchospore production in P. linearis from Ireland are 20degreesC and long days (16: 8 h), and suggest that intertidal conditions in summer are ideal for growth of this phase in the wild.