Nucleotide-diphospho-sugar transferases represent, in terms of quantity, one of the most important groups of enzymes on Earth, yet little is known about their structure and mechanism. Such a transferase, the spsA gene product involved in the synthesis of the bacterial spore coat in Bacillus subtilis, has been cloned and over-expressed in an Escherichia coli expression system. Crystals have been grown, using PEG 8000 as a precipitant, in a form suitable for high-resolution X-ray analysis. They belong to space group C2221, with unit-cell dimensions a = 42.4, b = 142.0, c = 81.4 A and with one molecule of spsA in the asymmetric unit. The crystals diffract beyond 1.5 A using synchrotron radiation.