Abstract X-ray diffraction patterns have been obtained from stretched fibres of whole carrageenin and its κ- and λ-components. The results establish structural differences between the two components, but the proposed fibre period in all three materials is 25.2 A. The κ-carrageenin molecule is probably branched. Along the main chain, the fibre period appears to contain two trisaccharide units each comprising two sulphated D -galactose residues linked α-1,3- and one 3,6-anhydro- D -galactose residue linked β-1,4-, while within each 25.2 A period a single side residue of 3,6-anhydro- D -galactose appears to be attached to the main chain through carbon 6 of a sulphated D -galactose residue. In λ-carrageenin the fibre period may represent three disaccharide units the majority of which are composed of two sulphated D -galactose residues linked α-1,3-. The long fibre period could be accounted for by a variation in the number of sulphate groups attached to the galactose residues or by the presence of side residues. λ-Carrageenin appears to have a monoclinic unit cell with a = 11.3 A; b (fibre axis) = 25.2 A; c = 6.0 A and β = 81°. This cell is traversed by one λ-carrageenin molecule. The unit cells for whole and κ-carrageenin are identical, and it is suggested that they probably monoclinic with β = 90° and axes a = 18.8 A; b (fibre axis) = 25.2 A and c = 11.4 A. In κ-carrageenin, four molecules traverse such a unit cell, while whole carrageenin it is traversed by two κ- and two λ-carrageenin molecules which alternate in sheets parallel to (100). The infrared spectra of the κ- and λ-components are similar. The potassium sensitivity of κ-carrageenin is explained in terms of its proposed structure.