When a calcareous red alga, Serraticardia maxima (Ohoshikoro) was illuminated in sea water containing H¹⁴CO^-₃, floridoside, floridean starch and trehalose that are major soluble carbohydrates in this alga were rapidly labelled in this order. During the dark culture following the illumination in sea water free of radiocarbon, the radioactivities and the absolute quantities of floridoside and trehalose remained almost unchanged while those of floridean starch were markedly decreased. On the other hand, feeding experiments with ¹⁴C-floridoside and ¹⁴C-trehalose on chopped algal fronds revealed that these carbohydrates were rapidly interconvertible in the intact cells. Moreover, the analysis of Ca¹⁴CO₃ deposited in cell wall during the feeding of the fronds with ¹⁴C-floridoside and ¹⁴C-trehalose indicated that these carbohydrates were rapidly consumed by respiration of the cells. From these results, it was concluded that these carbohydrates are storage products and in particular, floridoside and floridean starch seem to play similar physiological roles to sucrose and starch in higher plants, respectively. A small amount of laminitol was foundin this red alga, but the cyclitol appears to be an inert product.