The siphonous green alga Codium edule P. C. Silva (Bryopsidales, Chlorophyta) has the highest covering ratio among the macroalgae on the coral reef of Nanwan Bay in southern Taiwan, but its population in the subtidal region drastically decreases from July to September each year. The objective of this study was to determine whether the high temperature of summer could be the basis for this population decrease. Chlorophyll fluorescence measurements revealed that when the algae were incubated at 35°C (a temperature that can be reached in southern Taiwan during the summer), their photosynthetic activities were almost completely inhibited after about 8 h. The circadian rhythm of photosynthesis was disrupted at a temperature as low as 32°C. TEM studies showed that 4 h incubation at 35°C induced a decrease in turgidity accompanied by vacuole shrinkage and plasmolysis. The marked disintegrative changes, including damage to organelles, such as chloroplasts and nuclei, occurred after about 8 h, at which time central vacuoles collapsed and the cell interior was then filled with numerous small vesicles. Our results suggested that the rise in seawater temperature during the summer could be one of the major causes of the massive death of C. edule in the field.