In-situ measurements of specimens in research reactors and the health monitoring of commercial nuclear power plants are difficult because of high operating temperatures and the presence of radiation. One possible solution is to transmit ultrasonic guided waves into the harsh environment from a remote transducer. However, it is well known that large changes in temperature can significantly alter guided-wave propagation. The work presented in this paper examines how temperature, up to 700 K, influences guided-waves in a bar specimen of rectangular cross-section. The measurement setup consists of a bar specimen connected to a magnetostrictive transducer via a long wire waveguide. This allows the transducer to be located outside of the high temperature environment. Theoretical dispersion curve calculations as well as time-domain finite element models have been used to predict the behavior of group velocity. Preliminary results indicate that each wave mode has a unique response to temperature at a given frequency. Although higher order modes are generally more sensitive to temperature, the results also suggest the possibility of selecting wave mode and frequency to minimize the change in group velocity due to temperature.