The crystal structure of the racemate of rubidium ammonium hydrogen fluorocitrate dihydrate, RbNHaH(C6HaO7F).2H20, containing the isomer of fluorocitrate that inhibits the enzyme aconitase, has been determined. The configurations of the two isomers in the racemate are I R:2R and 1S:2S for 1-fluoro-2-hydroxy-l,2,3-propanetricarboxylic acid. The crystals are triclinic, space group PT, Z=2, with unit-cell dimensions a=8-458 (2), b= 10.910 (3), c=7.516 (2) A,, 0c= 102.80 ° (3), fl= 115.90 ° (3), 7,=80-50 ° (4). The observed and calculated densities are 1.92 and 1.925 g cm -3 respectively. The structure was solved from the Patterson map and refined to an R value of 0.072. All hydrogen atoms, except that on a carboxyl group of the anion, were located from a difference electron-density map. The fluorocitrate ion forms a tridentate chelate with a rubidium ion, the fluorine atom being shared by two rubidium coordination polyhedra. There is disorder of rubidium ions with ammonium ions to an approximate extent of 84 % and, corresponding to this, the fluorocitrate ion, also disordered to the extent of 84%, is presumed to pack so that in each case the fluorine atom is coordinated to the rubidium ion.