Canine lung tissue was investigated with short pulses of ultrasonic energy at frequencies of 2.4, 5.0, and 7.4 MHz. The objective was to determine the basic ultrasonic parameters of the tissue in the frequency range of present clinical ultrasonic apparatus and to assess the possibilities of using ultrasound as an aid in the diagnosis of diseases of the lung. Attenuation coefficients and reflection losses were measured for samples of fixed inflated tissue and collapsed freshly excised tissue. The reflection loss depended strongly on the degree of inflation and, unlike the results for other soft tissues, the reflection loss varied with the frequency. The attenuation coefficients increased linearly with frequency and were an order of magnitude greater than the values for other tissues. The fixation process did not substantially change the ultrasonic parameters. Because of the very high attenuation coefficients, the usefulness of ultrasound in diagnosis may be limited to conditions which are manifested at or near the periphery of the lung. The large variation of the reflection loss with frequency is felt to be potentially very important, although further experiments using samples with specific disorders are required to determine the sensitivity of the reflection loss to changes in the condition of the tissue.