The rheological behavior of aqueous dispersions of single-walled carbon nanotubes (SWNTs) in solutions of the cationic surfactant cetyl trimethyl ammonium bromide (CTAB) was investigated. The steady shear viscosity as a function of the applied shear rate was monitored in different concentrations of surfactant which correspond to different mesophases. We found that the presence of SWNTs had a dramatic effect on the behavior of the combined system not observed with other additives: a significant increase in the low shear-rate viscosity of SWNT dispersions, and shear thinning replacing Newtonian behavior were observed for CTAB concentrations below the onset of the surfactant hexagonal phase. As CTAB concentration increases the rheological behavior of the SWNT-CTAB system and the native CTAB solutions become more alike. We suggest that the origin of the observed phenomena is the good size-match between SWNTs and elongated CTAB micelles. Thus dispersed SWNTs may induce the formation of size-matched elongated CTAB micelles that further orient under the action of external shear. A similar effect was not observed in dispersions of multi-walled carbon nanotubes or carbon black particles, suggesting that the cooperative behavior is not invoked when significant size-mismatch exists between the surfactant micelles and the dispersed additives.