A bimorph piezoelectric flapper element (12) is mounted by one end in a valve casing (11) so that it extends into a valve chamber (10) as a cantilever. It tapers to the unsupported end on which a fluorosilicone blocking plate (16) is mounted. An outlet port (18) is formed by a tubular boss which projects into the valve chamber (10) and which tapers to a fine annular rim which is formed around the mouth of its bore at its tip. The blocking plate (16) is normally held by the flapper element (12) seated on the tip of the tubular boss to close the outlet port (18), the tip making an indentation in the fluorosilicone whereby to form a "bite" seal. The blocking plate (16) is unseated by the application of a low voltage to the bimorph piezoelectric element (12). The valve is suitable for hydraulic fluids and may be operated by optical power.