A marking instrument, e.g. a fiber tip pen, roller ball pen, or highlighter pen, has a feed device which conducts marking liquid from the reservoir chamber (17) to the marking tip supported by an elastomeric diaphragm (6) which partly confines the reservoir chamber and forms a valve with the feed device so that when tip is applied against a surface the feed device retracts causing the diaphragm (6) to pressurize the liquid in the reservoir and open the valve whereby liquid is pumped to the tip.