The transcription factor SinR, a pleiotropic regulator of late growth processes in Bacillus subtilis, has been crystallised as a complex with its antagonist SinI, in a form suitable for structural analysis. The SinI:SinR crystals diffract X-rays generated from a rotating copper anode source to 2.3 A spacing and a complete native dataset has been collected to this resolution limit. The space group of the crystals is P3(1)21 (or its enantiomorph P3(2)21) with cell dimensions a = b = 60.76 A, c = 87.79 A. Assuming that there is a single SinI:SinR heterodimer in the asymmetric unit, the crystals have a Vm of 2.53 A3.Da-1.