In this paper we consider the control problem for a class of partially observed deterministic systems governed by nonlinear differential equations with fuzzy parameters. Using Takagi–Sugeno fuzzy model, we propose a linear (fuzzy) controller, driven by the output process, for controlling the system. Further, using calculus of variations, we have developed a set of necessary conditions on the basis of which optimal control can be determined. Based on these necessary conditions we have proposed a numerical algorithm for computing optimal control along with some numerical simulations to illustrate the effectiveness of the proposed (fuzzy) control scheme.