A vortex-diode check valve comprises a short cylindrical working chamber with an axial inlet port entering axially through a first end wall at a short distance from the central portion of a second end wall and a tangential outlet port exiting tangentially through a circular side wall. The ratio of fluid flow resistances, between flows in the reverse and forward directions, is markedly increased by configuring at least one end wall as a flexible resilient diaphragm, permitting relative motion between the second end wall and the axial inlet port.