An X-ray diffraction analysis of a barium-vermiculite shows it to have a triclinic unit cell with the following dimensions a = 5:33, b = 9.26, c = 12.47 A, e = 100.75 ~ /~ = 93.5 ~ and ,/= 90 c'. The proposed structure, adjusted in space group C1, has the exchangeable cations approximately over the ditrigonal holes of the silicate surface and on either side of a hexagonal network of water molecules extending over the middle of the interlamellar region. The interlayer material imposes a relative dis- placement of + b/4 on adjacent silicate sheets. The importance of these displacements, not previously found in layer silicates, is emphasized.