The use of 13C nuclear magnetic resonance spectroscopy has yielded well resolved spectra of agaroses from marine algae of the Pterocladia species and Gracilaria secundata, and from the porphyran and alkali-modified porphyran of Porphyra columbina. A complete assignment of all signals in the spectra of these four undegraded polysaccharides has been made on the basis of 13C n.m.r. spectra which have been reported for various galactose derivatives and agaro-oligosaccharides. General shifts rules for carbohydrates have been used. Conformational aspects of the work are briefly discussed.