An annulus filler, mounted to a rotor disc of a gas turbine engine and bridging the gap between two adjacent blades attached to the rotor disc, is disclosed. The annulus filler, formed from a polymer matrix composite material, includes an outer lid, defining an airflow surface for air drawn through the engine in an axial airflow direction, and a support structure, connectable to the rotor disc, to support the rear of 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. Under centrifugal loads, the opposing support walls resiliently deform. Each support wall has a concave rear edge. Each rear edge has a first curved section, a second curved section and a substantially straight section therebetween.