Chitosan solutions of the same 83% degree of deacetylation (DD) but different weight average molecular weights (M(w)s) (78-914 kDa) in 0.01M HCl containing different concentrations of urea (0-6M) were prepared. Intrinsic viscosity ([eta]) and weight average molecular weight (M-w) of chitosan were measured with a capillary viscometer and Light scattering, respectively. Mark-Houwink exponent a was used as the parameter of conformational index. The Mark-Houwink exponent a increased with increasing concentrations of urea. When solutions contained 0, 2, 3, 4, and 6M urea, the value of a increased from 0.715 to 0.839, 0.894, 1.000, and 1.060, respectively. This indicates the occurrence of urea-induced conformational transitions of chitosans. The break point shifted from 223 kDa in solutions containing no urea to 280 kDa in 2M urea solutions, to 362 kDa in 4M urea solutions and further to 481 kDa in 6M urea solutions. (C) 2000 John Wiley & Sons, Inc.