A washer for washing and dewatering of cellulose pulp is disclosed, comprising a rotatable drum with a plurality of outer compartments for the pulp to be washed defined by axial compartment walls distributed along the circumference of the drum, a stationary cylindrical casing that encloses the drum, whereby an annular space is defined between the casing and the drum, the annular space divided by means of longitudinal seals in the axial direction of the drum into zones for forming, washing and discharge of the pulp. At least two of the longitudinal seals are arranged such that, when the drum rotates, they meet compartment walls at shifted points in time with respect to each other. Preferably, all of these longitudinal seals are arranged along the circumference of the drum at different positions with respect to respective opposite axial compartment walls.