Solution rheology of cellulose in 1-butyl-3-methyl imidazolium chloride ([BMIm]Cl) is reported using oscillatory and steady shear for cellulose concentrations from 0.1 to 10 wt %, spanning the dilute, semidilute unentangled, and entangled regimes. Although pure [BMIm]Cl is a crystalline solid at room temperature with a melting temperature of 65 degrees C, all solutions prepared at 75 degrees C are transparent and visually homogenous at 25 degrees C, and these supercooled solutions, with of order 0.1 wt % water, show no sign of crystallizing for months in either calorimetry or rheology measurements, allowing the potential for room temperature solution processing of native cellulose, such as fiber spinning. The overlap concentration of our cellulose in ([BMIm]Cl) is 0.5 wt % and the entanglement concentration is a factor of 4 larger (2 wt%). For semidilute unentangled solutions (between 0.5 and 2 wt %), the specific viscosity, relaxation time, and terminal modulus exhibit concentration dependences eta(sp) similar to c(2), tau similar to c, and G similar to c, respectively, while for entangled solutions (between 2 and 10 wt %) we find eta(sp) similar to c(14/ 3), tau similar to c(2.3), and G similar to c(2.3), consistent with scaling predictions for neutral polymers in a theta solvent. However, failure of the Cox-Merz rule with steady shear viscosity larger than complex viscosity and the observed internal mode structure of dilute and semidilute unentangled solutions suggest that cellulose in ([BMIm]Cl) is not simply a flexible polymer in a theta solvent. (C) 2011 The Society of Rheology. [DOI: 10.1122/1.3553032]