  We study the propagation of a density wave in a magnetically trapped
Bose-Einstein condensate at finite temperatures. The thermal cloud is in the
hydrodynamic regime and the system is therefore described by the two-fluid
model. A phase-contrast imaging technique is used to image the cloud of atoms
and allows us to observe small density excitations. The propagation of the
density wave in the condensate is used to determine the speed of sound as a
function of the temperature. We find the speed of sound to be in good agreement
with calculations based on the Landau two-fluid model.
