In a flat bed, Fourdrinier-type, countercurrent washer for pulp, the opportunities for the development of foam are minimized by "wet" operation of the washer so that the pulp mat remains essentially full of liquid as it passes from each washing zone to the next. Special provision is also made for compensating for drainage at too slow a rate in one washing zone by bypassing some of the flow of washing liquid to that zone so that it is delivered to the zone upstream therefrom. An additional feature is the provision of controls over the flow of drained liquid from one or more of the receptacles therefor so that any solid particles floating on the top of that liquid in the receptacle are delivered to the countercurrent flow which ultimately reaches the evaporator.