This paper presents the results of an experimental and theoretical investigation into the fundamental mechanisms that govern ultrasonic propagation in fibre slurries. An experimental apparatus which measures the attenuation and velocity of ultrasound in slurries is described. Measurements on wood fibre slurries show, in contrast to previous work, that the effect of the fibre on the velocity of ultrasound is negligible. This observation led to the development of an isolated segment model, which can predict attenuation as a function of fibre properties and ultrasonic frequency. The theory assumes the fibres are isolated, isotropic, infinite cylinders. It accounts for scattering, heat conduction, viscous losses in the fluid, and relaxation processes in the fibre. Experiment and theory are shown to be in good agreement for some synthetic fibre slurries. In the more complicated case of wood fibre suspensions, the theory predicts attenuation to the correct order of magnitude and permits speculation about the effects of fibre properties.