A set of torsional resonators is used to characterize the linear viscoelastic behavior of complex fluids in the kilohertz range. The frequency dependence of the elastic and loss modulus of a hard sphere dispersion, electrostatically and electrosterically stabilized particles, worm-like micelles, polystyrene microgels, and polymer solutions is studied. The results are compared to theoretical predictions for these systems. The utility of the instrument for characterizing the high frequency theology of complex fluids is demonstrated. This is especially relevant for suspensions or dilute solutions and gets, where time-temperature superposition often fails and the relaxation spectrum is inaccessible from conventional oscillatory shear rotational rheometry. (C) 2003 The Society of Rheology.