Abstract: My invention relates to an extensible stairway capable of being shipped in a knockdown condition and erected with a minimum of labor and without special tools. The stairway has a pair of stair step stringer assemblies each stringer assembly having a pair of telescoping channels nesting slidably together that can be extended to a required height. The vertical load bearing side walls of said channels are provided with a series of registering groups of equally step spaced in-line bores arranged to lie in a substantially horizontal line when the stringer channels are placed in stair inclined position. A plurality of stair step assemblies having tread supports are adapted to be swivably mounted and clamped to said groups of in-line bores to maintain the treads in a horizontal position when the stringers are inclined forwardly to an ascent angle. A rotatable bracket is located at the upper end of each of the inside channels enabling them to be mounted in an inclined position to the upper level of a structure.