The rheological behaviors of clustered and disentangled multiwalled carbon nanotubes (MWCNTs) dispersed in a stable, non-reactive Newtonian epoxy fluid have been investigated. Suspensions of untreated and chemically oxidized MWCNTs in epoxy show strong rate-dependent behavior and long-time elastic response that is characteristic of a flocculated microstructure. Suspensions of disentangled MWCNTs in epoxy were prepared by a functionalization reaction with nitrobenzene, and show distinct rheological behavior that is attributed to the motion and rotation of the individual nanoparticles. Characteristic features of the disentangled MWCNTs in epoxy are a lack of low-frequency plateau in storage modulus and shear thickening behavior at high shear rate. The rheological behavior is discussed on the basis of continuum-level predictions for the motion of individual, semiflexible fibers under shearing flows. Implications of MWCNT disentanglement on fundamental study and applications of MWCNT-filled systems are discussed.