Abstract The qualitative and quantitative occurrence of low-molecular-weight carbohydrates (LMWCs) in the Florideophyceae was surveyed using 13C-nuclear magnetic resonance spectroscopy and high-performance liquid chromatography. Besides the joint occurrence of the heterosides floridoside and digeneaside in various Florideophycean orders (Ceramiales, Rhodymeniales, Gelidiales and Gigartinales), the disaccharide trehalose was detected only in several members of the Ceramiales. While some taxa of the latter order such as Aglaothamnion exhibited only trehalose, others, such as Delesseria sanguinea, showed trehalose together with digeneaside. The biosynthesis and physiological function of trehalose in red algae remains an open question. In addition, recent data from the literature indicate strong variation between phenotypic and genotypic formation of trehalose among red algal orders, and hence there exists some uncertainty to use this disaccharide as a chemotaxonomic marker. The occurrence of digeneaside in s...