In the field of biotechnologyand medicineit is of interest to modeland simulate metabolicprocesses. The usual methods to model metabolic pathwaysare chemicaldescriptions anddifferential equations. Moreover,the graph theoretical aspect is discussed and the developmentof expert systems is in process. In this paper wepresent the formalization of metabolic processes. Our formalization is based on the theory of formal languages. This formalization is called genetic grammarand represents an expansion of the Semi-Thue-System.