We have carried out precise measurements of sound velocity c for fluid mercury at high temperatures and high pressures. The temperature coefficient of the sound velocity at constant pressure is negative in the metallic region, but it becomes small and positive in the non-metallic region. Similarly, the pressure coefficient at constant temperature is positive in the metallic region, while it is negative in the non-metallic region. Moreover, there appears a hump in the temperature or pressure dependence of c in the metal-nonmetal (M-NM) transition range. These anomalous behaviors could be understood if we would assume a. hypothetical two-phase region in the M-NM transition range. (c) 2007 Elsevier B.V. All rights reserved.