The crystal structure of a prehnite crystal from Tyrol, Austria, (a = 4.646 ± 0.002 Å, b = 5.483 ± 0.002 Å, c = 18.486 ± 0.005 Å, Z = 2) has been refined in space group Pncm. This refinement verifies the general features of the model proposed in 1959 by Peng, Chou, and Tang and shows further that two aluminum atoms are restricted to one of two tetrahedral positions in the structure. Weak reflections violating both the n and c glide requirements were observed for the Tyrol material and for prehnites from several other localities. These reflections are interpreted as indicating further fractionation of the two tetrahedral aluminum atoms in the position which has multiplicity four in space group Pncm. This fractionation may take place in one of two ways with both oidering schemes leading to a reduction of the space group symmetry. One scheme results in a P2cm space group and the other in a P2/n space group. Prehnites from several localities appear to be composed of domains of both of these symmetries.