The patch-clamp technique was used to obtain information on the existence and properties of ion channels in giant protoplasts obtained from the Gram-positive bacterium Streptococcus faecalis. The membrane proved to contain a pore with numerous conductance states, ranging from 10 pS to several nanosiemens. Application of a slight pressure differential across the membrane resulted in the activation of the channel. The pressure sensitivity points to a relationship between this channel and one recently discovered in E. coli spheroplasts [(1987) Proc. Natl. Acad. Sci. USA 84, 2297-2301] suggesting that pores of this type might be widespread among prokaryotes.