Rod-shaped crystals of apocrustacyanin C1 have been grown under microgravity on the International Microgravity Laboratory (IML-2) NASA space shuttle mission using the vapour-diffusion set-up of the Advanced Protein Crystallization Facility (APCF). The crystals obtained under microgravity are compared with crystals grown simultaneously in ground control experiments in identical APCF reactors, and with those obtained in the laboratory. The degree of reproducibility of the results in microgravity was also tested. Statistically, the microgravity-grown crystals are larger and of better X-ray diffraction quality than those grown in the ground controls but inferior to the best crystals grown in sitting drops, in the laboratory. Diffracting crystals, the best to 2.3 A, were produced in seven out of the eight reactors in microgravity, whereas the eight ground control reactors yielded only one poorly formed crystal suitable for diffraction studies, which also diffracted to 2.3 A. The crystals belong to the space group P2(1)2(1)2(1) with two subunits per asymmetric unit.