Ultrasonic beams of frequencies in the range of 80 kc/sec. to 1000 kc/sec. emerge from a rectangular slit of known adjustable width into air, oxygen, or carbon dioxide. The disposition of the field in front of the slit is studied by the aid of a hot wire anemometer, which gives the angle of divergence of the beam and the intensity distribution across the main and the diffracted beams.The results obtained indicate that the laws of diffraction are valid so long as the gas is far removed from its anomalous dispersion region.CO2in the range between 100 and 200 kc/sec. shows a high degree of scattering with the result that the emergent beam loses its definition completely and becomes very diffused.