Abstract: An omnidirectional ultrasonic intrusion surveillance unit can be suspended or supported in a room or like closed environment so that there is space substantially all round the unit, permitting surveillance all around the unit, that is, over substantially 4.pi. steradians, simultaneously. Ultrasonic waves are generated by an oscillator and radiated via an arrangement of transducers in a plurality of directions in the space surrounding the unit. The reflected waves are received from substantially all directions in the space by a transducer system associated with the wave radiating transducers, and a detector is provided to sense variations in the received reflected waves. Amplified signals produced by the detector in response to any such variations are applied to a trigger unit having a predetermined reference level indicative of intrusion disturbance of the wave field, and an external alarm is placed in operative condition if the level is exceeded.