In the framework of an ESA-MAP project entitled XRMON, directional solidification experiments of Al – 20 wt% Cu with in situ and real-time X-ray radiography were carried out during Parabolic Flight campaigns. Parabolic flights offer successions of periods with different gravity levels, allowing the investigation of the impact of gravity level variations on the solidification microstructure formation. Directional solidifications of refined Al – 20 wt% Cu alloy were investigated in a dedicated apparatus for a wide range of cooling rates and a constant temperature gradient. X-ray radiography was successfully used to observe the microstructure evolution following the variations of gravity level. During the columnar growth of the refined alloy a sharp increase of gravity level provoked the sudden nucleation of numerous grains ahead of the front. The most potent explanation of this effect is the variation of the liquid undercooling ahead of solid/liquid interface due to the changes of hydrostatic pressure in the melt.