A plurality of castors for a piece of furniture, each castor including a casing, two wheels, and fastener elements for fastening to the piece of furniture. The casing is made up of two units, each made as a single piece, the first unit including a portion constituted by a portion of the wheel casing and a portion constituted by a portion of the swivel casing, the second unit including a portion constituted by another portion of the wheel casing and a portion constituted by another portion of the swivel casing. The two units are adapted to be associated with each other to form the casing in such a manner that the portions of the first unit and the portions of the second unit engage respectively one in the other, and elements are provided for securing the two units together.