Intrinsic viscosity-molecular weight relationships were determined for poly(hexyl isocyanate) in toluene and dichloromethane using a number of narrow-distribution samples covering a wide molecular weight range. The data were analyzed according to the Yamakawa-Fujii-Yoshizaki theory of viscosity of wormlike cylinders, yielding the following values for the persistence length q and molar mass M//L per unit contour length: q equals 41-34 nm and M//L equals 730-760 nm** minus **1 in toluene between 10 and 40 degree C and q equals 21 nm and M//L equals 750 nm** minus **1 in dichloromethane at 20 degree C. These parameter values are favorably compared with those reported for other solvent conditions for the same polymer.