In the frame of a study of the effects of microgravity on protein crystallization, tetragonal thaumatin crystals were, for the first time, prepared in agarose gel within the Advanced Protein Crystallization facility (APCF) on Space Shuttle mission STS-95 (October 1998). We have reported elsewhere that these crystals are of superior crystallographic quality, with regard to their diffraction intensity and limit as well as mosaic spread, when compared to control crystals grown on Earth [Lorber et al., Acta Crystallogr. (1999) D55, 1491]. The analysis of images taken during the growth of these crystals under microgravity indicates that their nucleation was more synchronous than that of control crystals which were prepared simultaneously on Earth under otherwise identical conditions. Nucleation occurred essentially in the bulk of the medium and crystal growth proceeded up to 2 times faster. Crystals prepared under microgravity had better optical properties than control crystals. Experimental results give evidence of the existence of zones depleted in protein located around growing crystals. Also, they are in favor of a correlation between the better diffraction properties of these crystals and the more favorable crystallization parameters in Space. The content of individual crystals was analyzed and the results of purity analyses are presented. The advantages of crystallization in a gel under microgravity and importance of image documentation for asserting growth cessation are discussed.