A tubular anatomical structure in a mammalian subject is occluded by directing ultrasonic energy from outside of the body. For example, a male subject may be sterilized by directing ultrasonic energy through the skin of the scrotum, onto the vas deferens. The focal spot of an ultrasonic transducer may be registered with the structure to be treated by means of guide members on a probe holding the transducer. The procedure is simple and can be entirely non-invasive. Apparatus for performing this and other treatments may be provided as a disposable unit, and the guide members may be arranged to hold a fold of tissue without substantially obstructing the sonic path from the transducer to the fold.