Abstract: A precast concrete, plastic-jacketed filter underdrain block for downflow or upflow filter units is disclosed. The plastic jacket additionally functions as a mold for the concrete. The plastic molds become an integral part of the finished filter underdrain block. Thus, relatively lightweight, inexpensive plastic molds can be fabricated and shipped to the filter construction site where they are filled with concrete, cured, and then placed in the filter unit to provide an underdrain system. The underdrain blocks may be equipped with handholds to facilitate the handling and placement of the blocks in the filter vessel.