A reciprocating downhole pump has a gas separator located at its bottom end. The separator forms a chamber that is expanded and contracted when the pump is reciprocated. Expansion and contraction of the chamber occurs by either the plunger reciprocating in the chamber or a piston coupled to the plunger reciprocating in the chamber. The chamber has an orifice therein so that during reciprocation fluid flows in and out of the orifice. The orifice is sized so as to subject the fluid to a pressure drop, wherein gas in the fluid is separated from the liquid.