The fermentative lactate dehydrogenase (LDH) of Escherichia coli is induced by low pH under anaerobic conditions. Both translational and transcriptional gene fusions to ldhA, which encodes the fermentative LDH, have now been made. Both types of ldhA-lacZ fusion were induced by low pH, but only in the absence of air. However, the translational fusions were consistently expressed at a five-to tenfold higher level than the transcriptional fusions, perhaps implying some post-transcriptional effect on ldhA expression. Introduction of arcB ::Kan decreased expression of both translational and transcriptional ldhA-lacZ fusions by three-to fivefold. Disruption of mlc, which encodes a repressor of several genes of the phosphotransferase system, almost abolished expression of ldhA. Disruption of csrA caused a moderate drop in expression of both operon and protein ldhA fusions, whereas insertional inactivation of csrB or glgA had the opposite effect. These effects are probably indirect, resulting from alterations in sugar accumulation versus storage. Mutations in ptsG, cra, fnr, narL, rpoS, osmZ, appY, ack/pta, aceEF, pfl and ldhA had no effect on expression of the ldhA fusions. ldhA was not induced by the membranepermeant weak acid benzoate, implying that it does not respond to the internal pH directly. Little pH induction was seen during growth on glycerol plus fumarate, suggesting that products of sugar fermentation are necessary for acid induction. Addition of succinate, acetate or lactate had no effect on ldhA expression. In contrast, pyruvate caused a two-to fourfold increase in expression of ldhA-lacZ. This accords with the idea that increased sugar metabolism indirectly induces ldhA.