It has been observed that, in the 1988 Shuttle Discovery experiments, crystals grew uniformly throughout the solution and also uniformly on a membrane in the experimental chamber. This was very different from those grown in laboratories on Earth, where the crystals grew only on the vertical membrane and then only at the bottom half of the membrane [Scaife et al., 1990>. We propose that the combined effect of gravity (g&vec;) and geomagnetic field (B&vec;), in the form of g&vec;¿B&vec; drifts of ions, controls the growth of crystals in aqueous solutions. Based on this theory, the observed distinct difference between space-grown crystals and those grown in laboratories on Earth can be reasonably explained. We have conducted concept-proof experiments on crystal growth under gravity and different magnetic fields in the laboratory. Our experimental results have confirmed the g&vec;¿B&vec; effect, and showed that the uniformity of crystal growth as well as the size and yield of grown crystals can indeed be controlled by the combined effect of gravity and magnetic field. This work is conducive to the understanding of formation of crystals in nature. Moreover, it may be based on to develop a practical technique for industrial growth of crystals.¿ 1997 American Geophysical Union