Mercurous chloride crystals hold promise for many commercial applications. This material, produced using physical vapor transport (PVT) growth methods, has unique optoelectronic properties making it suitable in many acousto-optic applications. Research is being undertaken within the NASA Center for Commercial Development of Crystal Growth in Space, to determine the potential of using microgravity to grow crystals, thereby producing reduced defect density and, thus, fewer scattering centers and improved acousto-optic behavior. A major indicator of this potential is the velocity field behavior which exists within an ampoule during growth. Circulatory behavior would be indicative of natural-convective-induced flows and, hence, of strong potential for microgravity-affected crystal quality. Laser-Doppler velocimetry methods are being developed to examine the flow of mercurous chloride vapor in situ. Measurements have been made of the velocity profile during PVT growth of mercurous chloride at temperatures of near 300 degree(s)C. The results to date are indicative of a natural circulation pattern but have been found to be strongly dependent on the geometric characteristics of the heating system. This paper describes the measurement system which has the capability of measuring velocities as low as 10-5 m/s. A calibration system developed and used to determine the accuracy of the LDV system at these low velocities also is described. Finally, the paper presents the results obtained to date in mercurous chloride crystal growth in cylindrical ampoules at 300 degree(s)C.