Abstract: A multi-stage fluid filter filters entrained particulate matter from a moving fluid stream at substantially constant fluid pressure and filtration efficiency during continuous on-line operation. The filter includes a housing having a fluid inlet and a fluid outlet, first and second stationary filtration elements positioned in the housing, each having a filtration medium therein from an upstream side to a downstream side thereof. The two elements are positioned in series for fluid flow from the downstream side of one of the two elements defined as a first filter stage for accumulating particulate matter on the filtration medium thereof to the upstream side of the other of the two elements defined as a second filter stage having a previously accumulated layer of particulate matter on the filtration medium thereof.