This paper introduces the notion of input- (output-) decoupling structural zeros for linear time-invariant systems, described by Rosenbrock's system polynomial matrices. This notion is based on the definition of polynomial system matrices having the same structure that is given in this paper. Namely the input- (output-) decoupling structural zeros are decoupling zeros that are present for every choice of the system parameters. The main results are derived using digraph theory. An example illustrates the procedure for detecting structural zeros.