We present measurements of the dynamical structure factor Sðq; ωÞ of an interacting one-dimensional Fermi gas for small excitation energies. We use the two lowest hyperfine levels of the 6Li atom to form a pseudospin-1=2 system whose s-wave interactions are tunable via a Feshbach resonance. The atoms are confined to one dimension by a two-dimensional optical lattice. Bragg spectroscopy is used to measure a response of the gas to density (“charge”) mode excitations at a momentum q and frequency ω, as a function of the interaction strength. The spectrum is obtained by varying ω, while the angle between two laser beams determines q, which is fixed to be less than the Fermi momentum kF. The measurements agree well with Tomonaga-Luttinger theory.