Sillenite structure compounds such as bismuth germanate (Bi 12 GeO 20 ) are potentially useful in a variety of optical device applications. The containerless float-zone technique was used to produce crystals of this material in a microgravity environment on-board the space shuttle flight STS-77, flown in May 1996. The elimination of the container and the absence of gravitational forces were expected to yield crystals possessing superior optical properties. The microstructure and optical characteristics of the space processed crystals are discussed and they are compared to terrestrial processed bismuth germanate.