Seasonal patterns in thallus length, biomass, reproduction, total biomass m-2 and size structure were monitored in populations of Dictyota cervicornis and Stoechospermum polypodioides on a shallow reef flat in the southern Red Sea. These tropical reef flats are exposed to extreme temperatures of about 34°C in summer and to temperatures of about 25°C in winter. Both species showed peaks in length, biomass and reproduction in winter; macrothalli were absent in summer. Thallus length, biomass and the proportion of reproductive thalli showed a strong negative correlation with seawater temperature. Young thalli first appeared in November, following a drop in seawater temperature from about 33 to 31°C. Macroscopic thalli had disappeared in May, when temperatures had reached the same values as those at the start of the growth season (33°C). When reef substrata collected in summer were kept at sub-ambient temperatures (29–31°C), macroscopic thalli of D. cervicornis developed. Size structure varied over time, probably because of non-synchronous development and tissue loss among thalli. Reproduction was size-dependent. We conclude that D. cervicornis and S. polypodioides have highly seasonal patterns of growth and reproduction related to the seasonal variation in the environment, especially temperature.