In this paper, the linear and nonlinear rheological behavior of semidilute aqueous solutions of a recently synthesized thermoassociative graft copolymer was investigated, as a function of temperature and polymer concentration. The polymer, namely CMC-g-PNIPAM, is based on a carboxymethylcellulose (CMC) backbone bearing thermosensitive poly(N-isopropylacrylamide) (PNIPAM) sidechains. The samples have been submitted to steady shear, oscillatory shear, and step-strain experiments, mainly at temperatures above the threshold temperature T-assoc to observe thermothickening. The linear and nonlinear theological data clearly show the existence of two temperature regimes, separated by a transition temperature T' > T-assoc. At temperatures below T', the solutions behave like a soft critical gel, corresponding to weak PNIPAM segregation. At temperatures above T', the solutions behave like a stiff critical gel, corresponding to strong PNIPAM segregation. (C) 2003 The Society of Rheology.