Rheological measurements have long been an invaluable technique in studying mechanical and structural properties of polymers. Measurements on dilute, noninteracting polymer solutions allow the determination of macromolecular structural information, such as molecular weight. This analysis has been complicated by molecular polydispersity; thus, average effects are more commonly reported. Here, we demonstrate polydispersity characterization for rod-like polymers like single-walled carbon nanotubes (SWCNTs). By extending the theory of the rheological behavior of rigid rods, we determine the distribution of length and bundle size in suspensions of SWCNTs by a simple rheological method. The method is based on measuring the viscosity of dilute suspended SWCNTs over a shear rate range spanning the Newtonian and shear-thinning regimes. We show that a log-normal distribution in length with minimal bundling accurately describes the viscosity measurements. This rapid new method yields the SWCNT length distribution while relying on bulk samples, which are less prone to artifacts. (c) 2014 American Institute of Chemical Engineers AIChE J, 60: 1499-1508, 2014