High-silica ZSM-5 crystals were grown in the microgravity environment (10 À3 ±10 À6 g) of the space shuttle from precursor solutions held unmixed until activation on orbit. The ¯ight crystals grown from untreated (not heat-treated) silica gel grade 12 had the intergrown disk morphology and were larger than the spherulitic aggregates of small elementary crystals observed for the terrestrial/control samples. The ¯ight and the terrestrial/control crystals grown from silica gel grade 12 heat-treated at 973 K prior to synthesis had similar intergrown disk morphologies. The similar morphologies of the ¯ight crystals grown from untreated silica gel and the terrestrial/control crystals grown from heattreated silica gel imply that the nucleation rate of ZSM-5 was reduced in microgravity. The results of isomerization of 2,4,4-trimethyl-1-pentene (2,4,4-TMP-1) to 2,4,4-trimethyl-2-pentene on crystals grown from untreated silica gel showed that the ¯ight crystals exhibited a lower external surface activity than the terrestrial/control crystals with an identical total external surface area ($0.6% conversion of 2,4,4-TMP-1 after 3 h on ¯ight versus $1.3% conversion on the terrestrial crystals). This indicated that the ¯ight crystals had a lower surface roughness than their terrestrial controls. An atomic force microscopy examination of the crystal surfaces con®rmed these results. In contrast, both the ¯ight and the terrestrial/control crystals grown from heat-treated silica gel had similar surface topographies and roughness.