The complex character of plastic deformation of the austenitic steel Fe22Mn0.6C is studied at room temperature with the aid of high-frequency local extensometry. It is shown that the plastic flow instability, associated with fluctuations of the flow stress, results from quasi-continuous propagation of deformation bands along the specimen axis. This propagation mode is dominant in the entire range of the applied strain-rate from 2.1 Γ 10 -5 s -1 to 10 -1 s -1 . Such behavior differs from that of various alloys deforming via dislocation glide under conditions of dynamic strain ageing (Portevin-Le Chatelier effect), which is characterized by a transition from a repetitive occurrence of static deformation bands at lower strain rates to a relay-race and, finally, quasi-continuous deformation band propagation at higher strain rates. The unusual behavior of the deformation bands bears evidence to a particular kind of instability in the investigated steel. A possible role of deformation twins in the observed dynamics of plastic instability is discussed.