The paper describes an approach to scheduling in a Flexible Semiconductor Manufacturing System. The application refers to the production of memories in a new module built in the SGS-Thomson plant in Catania (ITALY). The approach uses Petri Nets in which the concept of coloured marks has been introduced. Although the scheduling approach has been devised for a particular industrial application, it can be extended to other applications sharing similar features. The paper will focus on the scheduling approach and on the improvement of performance obtainable by its use.