An annulus filler is provided for mounting to a rotor disc of a gas turbine engine and bridging the gap between two adjacent blades attached to the rotor disc. The annulus filler is substantially entirely formed from a polymer matrix composite material. It has an outer lid which defines an airflow surface for air being drawn through the engine in an axial airflow direction, and a support structure which is connectable to the rotor disc to support the lid on the rotor disc. The support structure has two support walls extending from opposing lateral sides of the lid to an attachment strap for receiving a hook on the rotor disc, the attachment strap bridging the support walls. In use, under centrifugal loads, the opposing support walls resiliently deform to allow outward radial movement of the lid. Each support wall is thickened in a region neighboring the attachment strap.