The structure of the phosphorylated form of isocitrate dehydrogenase from Escherichia coli has been solved and refined to an R-factor of 16.9% at 2.5-A resolution. Comparison with the structure of the dephosphorylated enzyme shows that there are no large scale conformational changes and that small conformational changes are highly localized around the site of phosphorylation at serine 113. Tyrosine 160 rotates by 15 degrees, and there is a local rearrangement of water structure. There is an 0.2-A net movement of loop 230-234, and side chain shifts of 0.2 A root mean square for isoleucine 159 and lysine 199. The lack of large conformational changes, the observation of a possible isocitrate binding site close to serine 113, and the demonstration that the phosphorylated enzyme is unable to bind isocitrate suggest that this enzyme is inactivated by a direct electrostatic interaction between the substrate and the serine phosphate.