Flow birefringence (FB) of polymaltotriose (pullulan) in water and in dimethylsulfoxide (DMSO) has been studied. Pullulan has a high refractive index increment (Δn/Δc) in water but in DMSO it is low. Accordingly, in water the FB of molecules is mainly determined by the macroform effect and in DMSO by the intrinsic anisotropy of polarizabilities of the polysaccharide chain. The analysis of the dependencies of reduced FB values [n]/[η] on molecular weight and on Δn/Δc makes it possible to determine quantitatively the value of intrinsic segmental anisotropy of pullulan molecules: (α1 - α2) = +33 × 10-25 cm3. On the basis of this value the optical anisotropy of the glucopyranose ring was determined. © 1997 Published by Elsevier Science Ltd.