Abstract: Apparatus and a corresponding method for the protection of an ultrasonic transducer operating immersed in a liquid, such as in an ultrasonic nebulizer. Surface motion of the liquid is detected by analysis of the voltage across the transducer terminals, and this surface motion indication is combined with a transducer temperature indication to determine whether or not the transducer should be shut off to prevent destruction or damage due to overheating or absence of surrounding water. Once turned off, the transducer cannot be re-energized without the removal and reapplication of electrical power to the apparatus. Provision is made for allowing the transducer to remain on even when no surface motion is present, provided the temperature is below a certain value, to permit adjustment of the transducer circuit while the transducer is in operation. An alarm is actuated when the transducer is turned off to call attention to the high temperature or loss of surface motion.