Expecting that protein crystal growth in space would show us a new aspect of its crystallization process and might result in the improvement of crystal quality, we performed a series of growth experiments for bovine pancreatic ribonuclease S crystals under microgravity. For this purpose, the COSIMA2 system launched in September 1989 was utilized. In one of the eight crystallization chamber setups in the COSIMA2 experiment, a large "Z-form" single crystal appeared and we found that (a) the space-grown single crystal exhibited a higher fraction of significant X-ray diffraction spots than ground-grown crystals of approximately the same size and (b) the corresponding Wilson plots also tended to show a slightly lower overall temperature factor for the space-grown crystals, although the quality of the plot was not sufficient for quantitative evaluation. Thus, mainly based on the above observation (a), it is most likely that the internal order of the ribonuclease S crystal was in fact improved through the crystallization under microgravity.