Crystals of alcohol dehydrogenase from Sulfolobus solfataricus were grown in the Advanced Protein Crystallization Facility during the Life and Microgravity Sciences Spacelab mission on the US Space Shuttle. Large diffracting crystals were obtained by dialysis, whereas only poor-quality crystals were obtained by vapour diffusion. The quality of both the microgravity and ground-based crystals was analysed by X-ray diffraction. There was some improvement in terms of size and diffraction resolution limit for the microgravity crystals. However, the twinning observed in the Earth-grown crystals was also present for those grown in microgravity.