During directional solidification of immiscible (hypermonotectic) alloys itis theoretically possible to establish a stable macroscopically-planar solidification front, and thus avoid sedimentation. Unfortunately, convective instabilities often occur which interfere with the directional solidification process. In this paper, stability conditions are discussed and results presented from directional solidification studies carried out aboard NASA's KC-135 zero-g aircraft. Samples were directionally solidified while the effective gravity level was varied from approximately 0.01g for 25 s to 1.8g for 45 s. Dramatic variations in microstructure were observed with gravity level during solidification. © 1992.