Eight different species of the red algal genus Porphyra from different biogeographic regions in Europe, Africa, North America, Asia and Australia were collected over the course of the respective growth season and surveyed for the qualitative and quantitative occurrence of the low molecular weight carbohydrates, floridoside, D-isofloridoside and L-isofloridoside. Except for Porphyra saldanhae from South Africa, all species tested exhibited clear seasonal variations in their heteroside composition. Whereas Porphyra dioica and Porphyra umbilicalis from the North Sea showed highest carbohydrate concentrations in spring and summer, those of Porphyra leucosticta from Spain were highest in winter. The red algae studied exhibited highest concentrations of heteroside at different seasons, some in winter, others in spring or summer. The composition of the three compounds varied among the species studied. In P. columbina from Australia, L-isofloridoside was always quantitatively dominant, while floridoside was the major component in P. dioica. Usually D-isofloridoside was present in small concentrations, except in P. perforata from the Pacific coast of the USA where it occurred in equal concentrations with floridoside and L-isofloridoside. The results point to species-specific different enzymic activities of the underlying anabolic pathways. Correlations between the heteroside concentrations and various prevailing environmental parameters are detectable but do not reveal a general response of Porphyra. Data from the literature prove that numerous physical factors determine the growth patterns of various Porphyra species in different biogeographic regions. Of these, photoperiod, temperature and nutrients are the most important. Highest heteroside concentrations in the Porphyra species studied always coincided with enhanced growth, and this relationship determined that growth, though related to abiotic conditions, was also promoted by maximum carbohydrate content. The multiple physiological function of floridoside, D-isofloridoside and L-isofloridoside as osmolytes, compatible solutes and carbon reserves is discussed in relation to the environmental stresses that Porphyra species usually experience in their upper intertidal habitats.