Summary: The unit cell of okenite (CaO.2SiO2.2H2O) has been determined for a specimen from Bombay, India, using a combination of X-ray, electron-microscope, and electron-diffraction methods. It is anorthic with a 9·84, b 7·20, c 21·33Å., α 90·0°, β 103·9°, γ, 111·5°, elongation b, Z 9. These data are compatible with the goniometric results of Boggild (1922) and allow the latter to be interpreted.A specimen from Crestmore, California, which Eakle (1917) had described as okenite, was also examined. It was found to be a new species, having the same composition as okenite but distinguishable from it by its optical properties, X-ray powder data, and unit cell. The latter is anorthic with a 7·60, b 7·32, c 9·86 Å, α 111° 48′, β 86° 12′, γ 103° 54′, elongation b, Z = 3. Because of the relation to okenite, the name nekoite is suggested.