AbstractSedimentation constant and intrinsic viscosity were measured on purified tropocollagens extracted from earthworm‐cuticle and lathyritic ratskin. A cartesian diver viscometer was used to make viscosity measurements at small shear stress and to avoid the effects of surface forces. By comparing the experimental data with the hydrodynamic theories of wormlike‐coil of Ullman a value of 1300 Å has been assigned for the persistence length of these tropocollagens. Other factors which may affect the estimate are discu