Crystal structures of types II and III chlorothalonil, 2,4,5,6-tetrachloro-1,3-dicyanobenzene, (C8Cl4N2) were solved by applying Monte Carlo simulated annealing techniques to X-ray powder diffraction data and refined using the Rietveld method. Both types of chlorothalonil crystallize in monoclinic symmetry (space group P21 and two molecules per unit cell). Lattice parameters are: a=8.1615(18) Å, b=9.4191(19) Å, c=6.4728(14) Å, β=93.7307(64)° and V=497.8 Å3 for type II, and a=8.6003(10) Å, b=9.2382(11) Å, c=6.3024(7) Å, β=96.2152(60)° and V=498.5 Å3 for type III. The structures of both types of chlorothalonil are stacked by two coplanar molecular sheets paralleled to the b-axis. The adjacent two paralleled molecules in type II are on the same plane, while those in type III are on two different parallel planes.