- The red microalga Porphyridium sp (P. sp) is encapsulated in a sulphated polysaccharide. The external part of this capsule dissolves in the growth medium. This extracellular polysaccharide is a heteropolyelectrolyte with molecular weight of ∼ 6 × 106 Da. The effect of solvent, counterion and pH on chain flexibility and structural features in dilute solution of the exopolysaccharide was investigated by intrinsic viscometry. From the dependence of the intrinsic viscosity [η] on ionic strength, it was estimated that the stiffness of P. sp polysaccharide chains is in the same range as that of xanthan and DNA. The effect of the counterion on [η] is found to be specific and dependent on the type and valence of the counterion. The polyelectrolyte behaviour of the polymer is confirmed by the decrease of [η] with the addition of salt without any observable order-disorder conformational transition in aqueous salt solutions in the commonly used range of ionic strength (0.01–1.0). At considerably lower ionic strength (<0.01) there is an indication of a transition in the P. sp polyion conformation, most likely reflecting a contraction of the polymer chain from a highly stretched to a stiff, wormlike chain. It is hypothesized from the overall dilute solution features that the P. sp biopolymer chain molecules adopt stiff ordered conformation in solution.