A frequency domain method based on the parametric transfer function concept is developed for the design of digital control systems with time-delayed continuous-time plants. The method allows to investigate the behavior of sampled-data systems in continuous-time in terms of input-output relations. An algorithm is presented for the design of an optimal digital controller that minimizes a quadratic cost functional under stochastic disturbances. The method is applicable to the design of digital controllers for arbitrary linear continuous-time plants.