A filtration system for a particulate-laden stream of gas utilizes a group of fabric bags within the upper chamber of a housing as the means by which particulate matter is filtered out of the gas. The gas inlet into a lower chamber of the housing is provided with an inlet guide that directs the incoming gas against the lower side of a porous diffuser located below the group of bags and spanning at least most of the separation zone defined by the bags. Passages through the diffuser are configured to present an array of nozzles so that the gas is subjected to a pressure drop across the diffuser before flowing to the bags. The nozzle effect causes the particulate-laden gas to spread out evenly on the lower, upstream side of the diffuser and pass through the diffuser at substantially equal velocity at all passages, thereby smoothing the flow and distributing it evenly to all bags.