A laminar separation bubble is formed in a region of adverse pressure gradient on a flat plate by a separating boundary layer that undergoes transition, finally leading to a reattached turbulent boundary layer. Linear amplification of steady threedimensional disturbances in the flow before separation and in the laminar part of such a separation bubble is studied by means of direct numerical simulation and an adjoint-based optimization technique suited to study spatial optimal transient growth. The steady disturbances develop as streaks following their excitation in the region of favorable pressure gradient. At separation and inside the bubble, numerical and experimental results show good agreement with theoretical predictions for the optimal disturbance. The growth rate ofthc steady disturbance is seen to possess a maximum around the spanwise wave length that was found to be a preferred one in the corresponding experiment.