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A 3 mm long double-polytype region was found in a ZnS needle. The region consists of a fine mixture of 47% of each of the polytypes 20L(5753) and 60R(5537)3 and about 6% of the polytype 60R(511553)3. The latter polytype is assumed to be the parent polytype transforming into the two others. It is the first time that traces of the parent polytype have been found in a double-polytype region. The formation mechanism of double-polytype regions is explained as a two-stage transformation process: expansion of grown-in stacking faults taking place at temperatures just below the hexagonal-to-cubic transition temperature, followed by a sudden formation and expansion of stacking faults at a lower temperature. It is suggested that a similar mechanism operates in the formation of single polytype regions and disordered ones.