The effects of molecular weight and urea on the magnitude of the Mark-Houwink exponent a and the relative stiffness parameter B of chitosan molecules in dilute solutions were analyzed. The results show that in solutions with ionic strengths between 0.01 and 0.3 M, the relative chain stiffness parameter B and the Mark-Houwink exponent a of chitosans whose molecular weights were between 22.3 x 10(4) and 91.4 x 10(4) fell between 0.143 and 0.152 and from 0.404 to 0.497, respectively; whereas for chitosans whose molecular weights were between 7.8 x 10(4) and 14.8 x 10(4) these values fell between 0.110 and 0.138 and from 0.653 to 1.009, respectively. Both results indicate that the stiffness and conformations of small molecular weight chitosans were more stiff and extended, respectively, than higher molecular weight ones, and that molecular weight-induced conformational transition occurred. Chitosans in solutions containing 4 M urea possessed a rod-shaped conformation in both molecular weight domains, and no molecular weight-induced conformational transitions occurred.