Summary: The honey-yellow apophyllite from Korsnäs, Finland, for which chemical, optical, and X-ray data are given, shows a complex twinning. An inne penetration twin is surrounded by two outer penetration twins. The twin laws have been established. The prism faces of the complex crystal are not (100) as conven-tionally taken for apophyllitebut (520). The spacegroup P4/mnc commonly accepted for apophyllite could not be confirmed. It is suggested that the mineral is only pseudotetragonal, the true symmetry possibly being orthorhombic with space group Pnn2 or Pnnm.