Indole is a small signalling molecule, produced by many species of bacteria, including Escherichia coli. It is made by the enzyme tryptophanase, which converts tryptophan into indole, pyruvate and ammonia. Indole has diverse roles in E. coli, including regulation of biofilm formation, acid resistance and pathogenicity. In these cases, E. coli responds to a low, persistent level of indole (0.5-1 mM), similar to the concentration found in an E. coli culture supernatant in stationary phase (typically 0.3-0.8 mM). Recently, it has been shown that much higher concentrations of indole (3-5 mM) inhibit cell division by acting as an ionophore to dissipate the membrane potential. However the biological relevance of such high concentrations, and therefore these aspects of indole signalling, has been questioned. This work has investigated the role of indole signalling during entry into stationary phase, when indole production is quickly upregulated. The viability of non indole producing mutants was compared to wild-type indole producing cells. In the short term indole producers suffered a growth disadvantage, but in the long term they were significantly more viable than their indole non-producing counterparts. The addition of 1 mM indole to the indole non-producing culture failed to complement the phenotype. A hypothesis was developed that a high rate of indole production during stationary phase entry leads to a transient, high concentration of indole inside the cell. This regulates cell growth and division via the ionophore mechanism. The validity of this indole pulse signalling hypothesis was tested by measuring cellassociated indole. For a brief time during stationary phase entry cell-associated concentrations reached 60 mM. Cell-associated indole represents an average of indole in the cytoplasm and the cell membrane. It was shown that indole has an approximately 100-fold greater affinity for the cell membrane. 60 mM cell associated indole is equivalent to approximately 4 mM in the culture supernatant, suggesting that the indole ‘pulse’ is sufficient to inhibit growth and cell division on entry into stationary phase. The indole pulse was dependent on the stationary phase sigma factor, SigmaS, which increases tryptophanase expression on entry into stationary phase. This increased tryptophanase expression occurs immediately prior to increased indole production. The end of the pulse seems to correlate with the exhaustion of tryptophan in the growth medium.