The space groups of different constituent domains in prehnite crystals from Farmington, Conn., were identified by single crystal X-ray diffraction, transmission and replication electron microscopy, and electron diffraction. A domain with space group P2cm is predominant in both X-ray and electron diffraction patterns. Sub-microscopic interpenetrational polysynthetic twinning of this domain simulates the space group Pncm. The latter can be observed by X-ray diffraction of apparently single macro-crystals, and by electron diffraction on twinned micro-crystals. A domain with space group P2/n is detectable in minor proportions by X-ray diffraction of macro-crystals, but has yet to be identified by electron diffraction.