The mechanism by which indole condenses with L-serine in the active site of tryptophan synthase was studied by the stopped-flow technique. The single turnover occurs by rapid binding of indole to the pre-formed enzyme--L-serine complex, followed by C--C bond formation, reprotonation of the alpha carbon carbanion of L-tryptophan, and its final release. The effects of isotopic substitution at C-3 of indole, of pH, and of the presence of indolepropanol phosphate on these processes were also studied. The mechanism of binding of indole complements the known mechanisms of binding of L-serine and L-tryptophan to give a detailed picture of the mechanism of catalysis. It invokes two competent species of enzyme--L-serine complexes, leading to a branched pathway for the central condensation process. The rates of dehydration of L-serine and reprotonation of the carbanion of L-tryptophan are probably limited by rearrangements at the active site. Analysis of absorption, fluorescence and circular dichroic spectra, as well as of published data on the stereoisomers obtained by reduction with borohydride, suggests that the rearrangement includes a reorientation of the pyridoxal phosphate C-4' atom. The mechanism provides a detailed framework for explaining all available information, including the activating effect of the alpha subunit on the reaction catalyzed by the beta 2 subunit.