A testing technique based on a new nondestructive ultrasonic device was applied to a whole avocado fruit to measure its internal changes during ripening and to assess its quality. The method was based on local measurements of ultrasonic wave velocity and attenuation, by means of two inclined ultrasonic probes, which were moved across the fruit peel. Multiple readings of wave amplitude and transit time over the peel of the fruit at different distances between the ultrasonic probes were used for calculation of wave attenuation and velocity. Nondestructive ultrasonic tests were performed daily on avocado (Persea americana Mill., cv ＇Fuerte＇) fruits, until full ripening was detected. Accepted methods were used for penetration measurements of firmness, and physiological tests of dry weight (DW) and oil content of the fruit tissue. Results of the ultrasonic tests were compared with those of the destructive penetration tests and the physiological tests. Average values of the ultrasonic wave attenuation could be correlated well with storage time and with the ripening process of the avocado. A linear model well represented the changes in attenuation of the ultrasonic signals in the fruit, while second-order polynomial expressions described the changes in the firmness of the fruit during storage at room temperature. The wave attenuation increased during storage while the firmness of the fruits decreased. The DW of the avocado and its oil content were correlated with its ultrasonic parameters and mechanical properties. The physiological results showed wide scattering among the data from individual fruits. A preliminary attempt to explain these phenomena is discussed. It is anticipated that changes in the ultrasonic parameters during storage may be used to evaluate firmness and oil content to evaluate the ripening process of avocado fruit.