Oscillatory and steady shear of cellulose in 1-ethyl-3-methylimidazolium methylphosphonate {[EMIm][P(OCH3)(H)O-2]} were studied for cellulose concentrations from 0.017 to 10 wt. %, spanning the dilute, semidilute unentangled and entangled regimes. The overlap concentration of our cellulose in [EMIm][P(OCH3)(H)O-2] is 0.065 wt. % and the entanglement concentration is 8x larger (0.5 wt. %). The concentration dependences of specific viscosity, relaxation time, and terminal modulus suggest that [EMIm][P(OCH3)(H)O-2] is a good solvent for cellulose. Compared with the cellulose/1-butyl-3-methyl imidazolium chloride ([BMIm] Cl) system, this native cellulose in [EMIm][P(OCH3)(H)O-2] has lower c* and c(e), and much higher extent of entanglement. A significantly wider than expected rubbery plateau for the entangled solutions and failure of the Cox-Merz rule suggest that cellulose in [EMIm][P(OCH3)(H)O-2] is not simply a flexible polymer in a good solvent. The relaxation mode distribution in the terminal regime is much broader for cellulose in [EMIm][P(OCH3)(H)O-2] compared with the same cellulose in [BMIm] Cl. The delocalized phosphonate anion likely binds to hydroxyl groups on multiple cellulose chains, acting as temporary crosslinks. (C) 2017 The Society of Rheology.