The design and operation of a simple gas circulating pump is described. This pump incorporates a piston with an O‐ring seal and hollow glass check valve and is capable of circulating gases over a large pressure range, 769–1 Torr. The pump could conceivably operate at lower pressure by changing the check valves. During pumping operations the leak rate was less than 1 &mgr; (for 400–500 cycles). The maximum pumping speed is about 110 cm3/sec although greater pumping rates can be obtained from larger pumps. The pump is designed so that it can be easily disassembled for cleaning.