The consequences of the choice of the modelling procedure on the design of a state variable controller are discussed at the example of a complex hydraulic drive, for whose description both a classical approach - through physical analysis - and a "black-box" identification method were used. Although physical modelling offers a better guide for pole assignment and a physical understanding of the possible model simplifications suggested by mathematical tools, experimental results show that for linear controllers with fixed coefficients the choice of the the procedure for the controller design is far more important than the choice of the modelling strategy. This can be easily explained with the poor use made by a linear controller of the supplementary information provided by the nonlinear model. General suggestions for this class of problems are derived. © 1990.