A close flyby of Saturn's satellite Rhea by the Cassini spacecraft on 26 November 2005 can be used to infer its mass and quadrupole gravity moments. From these data we derive a mean density of 1233 ¿ 5 kg m-3 for Rhea and an axial moment of inertia consistent with an undifferentiated interior. The small density implies an interior made up of about 25% rock-metal and 75% water ice by mass. The moment of inertia implies a homogeneous mix of these constituents with some compression of the ice and the transition from ice I to ice II at depth.