As part of a study of the effects of microgravity on protein crystallization, the growth of tetragonal crystals of thaumatin was monitored by CCD-time-lapse video in environments where convection is negligible. In Space Shuttle missions entitled United States Microgravity Laboratory-2 and Life and Microgravity Sciences, free interface diffusion and dialysis techniques were utilized to grow crystals in the advanced protein crystallization facility (APCF). Ng et al. (Acta Crystallogr. D 53 (1997) 724) have shown that the crystals recovered in these experiments are of superior crystallographic quality (at the level of their diffraction intensity, resolution, and mosaicity) with regard to earth controls. Here, the number of crystals, their size, growth rate, and protein solubility in microgravity were compared with data of dialysis experiments performed in parallel on earth. Image analysis shows that in microgravity about one-quarter to half of the crystals have nucleated and grown in the bulk of the solution, the remaining being attached to the walls of crystallization vessels. The growth of free-floating crystals was 2.5 times faster, has resulted in 15-fold larger crystals, and consumed more protein than that of attached crystals in earth controls. Distances between immobile free-floating crystals in microgravity were related to the size of the latter. Experimental results are in favor of a correlation between more favorable growth parameters in microgravity and better diffraction properties. The displacements of free-floating crystals at various velocities and in various directions on unrelated trajectories are indicative of drift and stirring motion. On the basis of an overview of reactors monitored on four APCF missions some forces causing motion are proposed. Advantages of crystallization in microgravity are discussed and recommendations for future experiments given.