The rheology of five fractions of the microbial polysaccharide xanthan has been studied in semidilute aqueous solution. Samples covering a 3-fold range of molecular weight ((0.46-1.20) X 10(6) g mol-1) and a 3-fold range of concentration (1.0-3.0 g dL-1) have been investigated. The samples were heated for 20 min at 121-degrees-C in the presence of 0.10 M NaCl and cooled to room temperature for measurement. Xanthan samples prepared in this way exhibit large increases in steady shear viscosity, dynamic viscosity, and dynamic storage and loss moduli relative to the unheated control samples. Procedures are presented for bringing much of the data onto rather well-defined master curves that disclose clear systematic differences between the behavior of autoclaved and unheated samples and between xanthan samples of high and low molecular weight. The heat-treated xanthan samples display viscoelasticity and pseudoplasticity that rival or exceed those shown at similar concentrations by the polysaccharide hyaluronic acid (HA), widely employed in ocular surgical techniques for its desirable rheological properties. The development of HA-like rheology in xanthan is interpreted here in terms of tenuous network formation involving junction zones between xanthan chains that are based on the duplex motif of the native xanthan double strand. Under the conditions of added salt described, autoclaving at 121-degrees-C is postulated to disrupt the native duplex, which re-forms on cooling of the highly interpenetrating semidilute polymer coils to form additional duplex network junction zones. Extreme pseudoplasticity arises from the ready disruption of these junction zones by shear. The higher molecular weight controls also exhibit many rheological characteristics in common with the autoclaved samples. These similarities are interpreted in terms of the existence of a significant proportion of nonequilibrium duplex structure, and hence more extensive network development, in the higher molecular weight controls than is present in their lower molecular weight counterparts.