SummaryWe report the characterization of themsbAgene, isolated as a multicopy suppressor of the HtrB temperature‐sensitive phenotype. ThemsbAgene maps to 20.5 min on theEscherichia coligenetic map and encodes a protein with an estimated molecular mass of 64460 Da, with the properties of an integral membrane protein. The amino acid sequence of MsbA is very similar to those of the family of ATP‐dependent translocators, which includes the haemolysin B protein ofE. coliand the mammalian multidrug resistance (MDR) proteins. Mutational analysis ofmsbAindicates that it may form an operon with a downstream gene,orfE, and that both of these genes are essential for bacterial viability under all growth conditions tes