Zeolites A and X were grown hydrothermally in the microgravity environment (10(-3)-10(-6) g) of the space shuttle. It was hypothesized that this quiescent environment would lead to a means of producing "large" zeolite crystals having a high degree of crystalline perfection, and/or to a means of controlling defect concentration and location. Zeolite crystallizations have been carried out on four separate space shuttle missions: STS-40, USML-1, Spacehab-1, and USML-2. Preliminary results (STS-40) indicated that premixed solutions produced equal-sized intergrown crystals, and suggested that a "pre-nucleation" process had occurred before launch, All successive crystallizations were performed by mixing the precursor solutions on orbit, When nucleation was controlled using the aluminum complexing agents triethanolamine or 2, 2-bis(hydroxymethyl)-2,2', 2"-nitrilotriethanol, crystals synthesized in microgravity were similar to 25-70% larger in Linear dimension compared to their ground-based controls. The crystals grown in orbit had different nucleation/growth histories as indicated by their particle size distributions (PSDs). They frequently had different morphology (i.e., prominence of the {110} faces on zeolite A grown on earth vs, those grown in orbit) than did crystals grown from identical solutions, identically processed on earth. Based on the volume-normalized PSDs, similar to 15-40% fewer nuclei for zeolite A and similar to 40-60% fewer nuclei for zeolite X were observed in orbit,