- Porphyridium sp (P.sp) is a red marine microalga which is encapsulated within a charged polysaccharide envelope. The external part of this envelope dissolves in the growth medium. This mucilaginous polymeric material harvested from the medium is an anionic heteropolysaccharide containing both carboxyl and half-ester sulfate groups. Aqueous preparations of P.sp polysaccharide yield highly viscous samples with unique rheological properties which may be of commercial value. Rheological studies of aqueous preparations have been combined with X-ray diffraction studies of oriented fibers in order to investigate the conformation and interactions of the macromolecules. The X-ray fiber diffraction suggests the presence of an oriented single two-fold helical structure of pitch 1.6 nm, indicating that the P.sp. polysaccharide contains extended regions with a regular chemical repeat. Aqueous preparations exhibit relatively high viscosity at low polymer concentrations. Within the investigated concentration range the preparations show marked shear thinning behavior with no evidence for a Newto-nian plateau. Small deformation mechanical studies of 1% show evidence for elastic behaviour indicative of a weak gel network. An unusual feature of these samples is a reversible sharp transition upon heating from a weak to a strong elastic network.