Abstract A high purity Si x Ge 1 −x alloy ( x 0=0.89) 48 mm in length with a maximum diameter of 25 mm was grown by the Czochralski method. Development of the cellular structures was evaluated by observing growth striations in the crystal by means of X-ray topography and optical microscopy. Temporary and fatal cellular structures were detected. Two possible factors were distinguished as being responsible for the origin of the cell formations: an enhancement of the effective growth velocity and a gradual reduction of Si composition. Both factors lead to the occurrence of constitutional supercooling beyond the critical relation between growth velocity and alloy composition.