Time-resolved X-ray imaging with monochromatic synchrotron radiation enabled in situ observation of dendritic growth and detachment of the fragmented dendrite arms during unidirectional solidification of Sn-21 mass% Bi alloys. A mold with a re-entrant channel was used for the unidirectional solidification. At the beginning of the solidification in the re-entrant channel, detachment of the fragments was observed after the interface broke down and formed the dendrites. In addition, the detachment was observed after the dendrites passing the re-entrant corner laterally grew at rather high growth rate. In both occasions, the detachments followed deceleration of the growth rate and some of them resulted in the stray crystals. The observation showed that the detachment of the fragmented arms could be a source of the stray crystals in the metallic alloys. (C) 2003 Elsevier B.V. All rights reserved.