The behavior of brome mosaic virus in solution as a function of physico-chemical conditions has already been characterized by Small Angle X-ray Scattering studies. The most striking result was that the precipitates induced by the addition of polyethylene glycol were in fact made of microcrystals. This result was reinvestigated on the ID2 beamline at ESRF (Grenoble, France) to determine whether there was an amorphous state before the organized one, and measure the necessary period of latency for the microcrystals to form. The stopped-flow device associated to the high brilliance of the beamline enabled us to characterize the growth of the diffraction peaks as a function of time.