The protein apocrustacyanin C1 has been crystallized by vapour diffusion in both microgravity (the NASA space shuttle USML-2 mission) and on the ground. Rocking width measurements were made on the crystals at the ESRF Swiss-Norwegian beamline using a high-resolution psi-circle diffractometer from the University of Karlsruhe. Crystal perfection was then evaluated, from comparison of the reflection rocking curves from a total of five crystals (three grown in microgravity and two earth controls), and by plotting mosaicity versus reflection signal/noise. Comparison was then made with previous measurements of almost 'perfect' lysozyme crystals grown aboard IML-2 and Spacehab-1 and reported by Snell et al. [Snell, Weisgerber, Helliwell, Weckert, Holzer & Schroer (1995). Acta Cryst. D51, 1099-1102]. Overall, the best diffraction-quality apocrustacyanin C1 crystal was microgravity grown, but one earth-grown crystal was as good as one of the other microgravity-grown crystals. The remaining two crystals (one from microgravity and one from earth) were poorer than the other three and of fairly equal quality. Crystal movement during growth in microgravity, resulting from the use of vapour-diffusion geometry, may be the cause of not realising the 'theoretical' limit of perfect protein crystal quality.