The Krebs citric acid cycle, a central pathway of intermediary metabolism, generates ATP, reducing power, and biosynthetic intermediates. This chapter provides a list of enzymes of the Krebs cycle and the genes that encode them. In Bacillus subtilis, isocitrate dehydrogenase (IDH) reduces NADP+, but in other organisms IDH is an NAD+-reducing enzyme. Among the enzymes from gram-positive bacteria, Streptococcus mutans IDH is unusual; it resembles the E. coli IDH in that it lacks the extra loop of amino acids found in B. subtilis IDH. The loss of aconitase activity causes a severe block in SpoOA~P-dependent gene expression and near-total blockage of sporulation at stage 0. In large measure, this defect can be attributed to accumulation of citrate, both inside the cells and in the medium. Citrate is a chelatot of divalent cations; in this case, Mn2+ and Fe2+ are the relevant cations. The B. subtilis citH gene encodes the sole malate dehydrogenase (MDH) of this organism, as judged by total loss of MDH enzyme activity and at least partial aspartate auxotrophy in a citH null mutant. The citB gene is upstream of and divergently transcribed from cotK in all Bacillus spp. examined. The enzymes for the dicarboxylic acid part of the cycle probably appeared very early in evolution, since they are found in species from all branches of life. The genes for isopropylmalate isomerase (leuC) and isopropylmalate dehydrogenase (leuB) are close homologs of the citB and citC genes, respectively, and the reactions catalyzed by the respective enzymes are chemically analogous.