Abstract: Apparatus for detecting the presence of gas bubbles passing through an infusion tube (10). Sensing means (24, 26) responsive to fluid flow through the infusion tube develop a signal having a characteristic representative of a gas bubble to start a counter (28) which counts timing signals which also drive the pumping means (12, 14, 22) which force infusion liquid through the infusion tube. After a predetermined number of timing signals have been counted corresponding to a gas bubble having a size which may cause injury to a patient being infused, an alarm (30) is actuated.