We prove a maximum principle of Pontryagin type for the time optimal control of a hybrid system. The system is nonlinear and consists of a controlled coupled ODE/PDE. The control input acts via the boundary and the interior of the domain. Systems of this type frequently arise in modeling population dynamics in a contaminated environment. For the regularization we use Ekeland＇s variational principle along with a singular perturbation technique. For this purpose we introduce a new trajectory on an additional exterior domain, whose size may be considered as a singular perturbation parameter. It turns out that the maximum principle is stable with respect to the size of the additional exterior domain. The technique allows us to obtain necessary optimality conditions without involving measure boundary data.