In patch-clamp experiments on giant protoplasts of the Gram-positive bacterium Bacillus subtilis, membrane stretch resulted in an initial transient collapse of the membrane resistance, after which stretch-activated, voltage modulated, high-conductance channels could be observed. The channel open probability increased exponentially with applied suction and positive voltage, as a result of variations of both the mean open and the mean closed times. The substate structure and other characteristics of the electrical activity suggested the presence of a family of pores exhibiting cooperative behavior. A role in osmotic protection is suggested. In the intact bacteria, the pores may be part of an unidentified envelope apparatus, having other functions as well.