Fairly narrow fractions of tri-O-heptylcellulose (THC) covering a wide range of degrees of polymerization (DP) were prepared and studied by viscometry and light scattering to determine the wormlike model parameters of the polymer to be q = 43.2 exp(-0.0050T) nm and M(L) = 880 nm(-1), where q is the persistence length, M(L) is the shift factor, and T is the absolute temperature. The same fractions were studied with their thermal properties to determine the anisotropic-isotropic transition temperature T-i and transition entropy Delta S as a function of DP. Both T-i and Delta S increased with increasing DP to approach a limiting value for large enough DP. These trends were favorably compared with the predictions of the statistical models with the above-noted parameter values. It was shown that the main factor that causes the thermotropic transition and the chain-length dependence of T-i in bulk THC is the temperature-dependent flexibility of the polymer.