A washer for washing and dewatering cellulosic pulp is disclosed, comprising a rotary drum with a plurality of external compartments defined by axial compartment walls distributed along the drum's circumference, a stationary cylindrical casing enclosing the drum, whereby a ring-shaped space is defined between the casing and the drum and where longitudinal seals divide the ring-shaped space into a forming zone for forming the pulp, at least one washing zone for washing the pulp at overpressure, and a discharge zone for feeding out the washed pulp, and the filtrates from different washing stages are collected in a peripheral valve located at the drum's end wall, the filtrate stages are separated by seals in the valve, and at least some of the valve seals are displaced in relation to the corresponding longitudinal seal, as seen in the direction of rotation of the drum.