The weight-average molecular weight M-w, z-average radius of gyration R-g, and second virial coefficient A(2) have been determined between 15 and 52 degrees C for dilute aqueous solutions of methylcellulose (MC) with three different molecular weights and constant degree of substitution (DS) of 1.8 using static light scattering. These measurements, conducted within 1 h of heating the homogeneous solutions from 5 degrees C, reveal that the theta temperature for MC in water is T-0 = 48 +/- 2 degrees C, with A(2) < 0 for T > T-theta, indicative of lower critical solution temperature (LCST) behavior. However, after annealing a solution for 2 days at 40 degrees C evidence of high molecular weight aggregates appears through massive increases in the apparent M-w and R-g, a process that continues to evolve for at least 12 days. Cryogenic transmission electron microscopy images obtained from a solution aged for 3 weeks at 40 degrees C reveal the presence of micron size fibrils with a diameter of 16 +/- 4 nm, structurally analogous to the fibrils that form upon gelation of aqueous MC solutions at higher concentrations and elevated temperatures. Growth of fibrils from a solution characterized by a positive A(2) indicates that semiflexible MC dissolved in water is metastable at T < T-theta, even though the solvent quality is apparently good. The minimum temperature required for MC solutions to aggregate is estimated to be 30 degrees C, based on the rate-independent gel-to-solution transition determined by small-amplitude oscillatory shear measurements conducted while cooling 0.5 and 5.0 wt % solutions. These results cannot be explained based solely on separation into two isotropic phases upon heating using classical Flory-Huggins solution theory. We speculate that the underlying equilibrium phase behavior of aqueous MC solutions involves a nematic order parameter.