A filter screen assembly is shown for filtering drilling mud used during oil and gas well drilling operations. The filter screen assembly is installed at the surface of a well on the discharge side of the mud pump at ground level. The filter screen assembly is installed upstream of the rig swivel, rotary hose and Kelly and downstream of the discharge from the mud pump. Since it is located at ground level, it does not pose the safely hazard associated with elevated components. It does not need to be removed and reinstalled when additional segments are added to the drill string and is always easily accessible for routine maintenance or replacement.