Members of the GTPase superfamily are extremely important in regulating membrane signalling pathways in all cells. This review focuses on membrane-associated GTPases that have been described in prokaryotes. In bacteria, LepA and NodQ are very similar to protein synthesis elongation factors but apparently have membrane-related functions. The amino acid sequences of FtsY and Ffh are clearly related to eukaryotic factors involved in protein secretion. Obg and Era are not closely related to any GTPase subgroup according to amino acid sequence comparisons, but they are essential for viability. In spite of similarities to well-studied eukaryotic proteins the signalling pathways of these cellular regulators, with the exception of NodQ, have not yet been elucidated.