Abstract: A spiral staircase comprising a central hollow column of circular cross-sectional shape and a plurality of treads, said column having a plurality of apertures in its wall spaced spirally, each said tread having a curved locking plate at its inner end which lies contiguous with the outer face of the column and a locking pin projecting radially inwardly from the locking plate and extending through a respective aperture in the wall of the hollow column, each said locking pin having surfaces defining an upwardly extending slot which accommodates portion of the column wall, one of said slot surfaces bearing against the inner face of the column thereby inhibiting tilting of that tread.