An ultrasonic probe having structure for receiving a puncturing cannula therethrough, the probe including a substantially rectangular parallelepiped-shaped support member, the height of the member being relatively much less than the length and width, a plurality of ultrasonic transducer elements arranged in at least one row closely adjacent to one edge of the bottom surface of the support member, cable structure for individually connecting said transducer elements to an external processing unit, and a slot for guiding the puncturing cannula, the slot opening into a first face of the parallelepiped-shaped member perpendicular to the bottom surface and adjoining said row of transducer elements, and said slot being centrally positioned relative to the row of transducer elements. Preferably the depth of the slot is only slightly greater than the diameter of the cannula and the width of the slot permits substantially no lateral movement of the cannula parallel to the row.