Abstract A theory is advanced under which chemo-, aero-, and phototactic reactions of motile bacteria are subsumed. It is put forward that these are, in fact, examples of behavioral responses triggered by a critical alteration of the membrane potential. According to the argument advanced here, the cytoplasmic membrane serves as a primitive receptor-transmitter sensing device, and in any case where a stimulus gradient is encountered, depolarization above a threshold value occurs. An over-compensating hyperpolarization and a resulting flagellar reorientation follows, thereby leading to an immediate directional change in translational motion. The theory does not invoke an elaborate internal flagellar co-ordination mechanism to account for observed response patterns.