A turbine stage for a turbomachine, the stage comprising a rotor wheel mounted inside a sectorized ring carried by an outer casing, and a sectorized nozzle fastened to the casing downstream from the wheel, each ring sector having a downstream end that is held bearing radially outwards against a cylindrical rail of the casing by means of the nozzle, the casing rail including an annular row of through radial notches reducing the areas for heat exchange by conduction both between the casing and the ring sectors and between the casing and the nozzle.