An analysis of the selective withdrawal features of a two-dimensional skimmer wall is developed based on the concept of a double critical flow section in the two-layered stratified fluid system. The theory predicts the withdrawal ratio as a function of two governing parameters, a densimetric Froude number and a layer depth ratio. The skimmer wall design condition of incipient withdrawal as derived by HARLEMAN and ELDER (1965) is included as a special case. Good qualitative agreement with available experimental data is found. An “effectiveness” diagram displaying the fully effective, partially effective and non-effective regimes of skimmer wall operation is developed.