Diffraction effects in the ultrasonic field of a circular piston source are discussed with particular reference to the measurement of ultrasonic absorption in liquids by the pulse technique at frequencies of 1 Mc/sec and above. A relationship given by Williams for the acoustic pressure, integrated over a “measurement circle” equal in area to and coaxial with the source, is used to develop a new formula representing the apparent attenuation due to diffraction. This is simpler and more general than the corresponding formula derived by Williams. It covers all distances between source and detector larger than the radius of the source irrespective of the ultrasonic wavelength in the medium. Measurements made in water at frequencies of 1 and 3 Mc/sec confirm the theory qualitatively.