An ultrasonic interferometer with high sensitivity is used to measure acoustic velocities at 0°C in gases, as the wavelength approaches the mean free path of the molecules. Measurements are made of velocity as a function of pressure down to about 2 mm of Hg, the frequency being kept constant at approximately 970 kc. The following increases in velocity (at the lowest pressures) over that at standard conditions were observed: argon 27 percent, nitrogen 16 percent, oxygen 20 percent, and dryCO2‐free air 7 percent.