Membrane strip diffusers are disclosed, useful for example in aerating wastewater in activated sludge plants. These diffusers have membranes, diffuser bodies and gas conduits elongated in the same general direction. Such conduits may be attached to or formed integrally with the diffuser bodies. Gas chambers form beneath the membranes when they inflate, and these are separate from but communicate with the gas conduits, e.g., through passageways distributed along the lengths of the membrane supports. Preferably, the passageway flow cross-sections are small, thus tending toward uniform distribution of gas along the membrane's length. Ways to edge- and end-seal the membranes to the diffuser bodies are also disclosed.