Escherichia coli accumulated taurine at high osmolarity via the ProU and ProP transport systems. Taurine accumulation was shown to be osmotically active as it displaced cytoplasmic K+. In contrast to betaine and proline, taurine only modestly enhanced the growth rate of E. coli at high osmolarity and only if the cell was unable to synthesise trehalose. These studies show that taurine cannot be used as a tracer for the extra-cytoplasmic space of bacteria grown at high osmolarity.