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Summary1 Pressurized ventilation, which increases gas exchange between aerial and submerged plant parts, has been found in various emergent macrophyte species. We investigated the potential for this mechanism to aect growth, morphology and biomass allocation in Phragmites australis in glasshouse experiments. 2 Inhibition of pressurized ventilation by perforation of stems above the water surface resulted in decreased oxygen concentrations in stem bases and rhizomes. Perforation caused little mechanical damage. 3 Allometric methods were used to evaluate treatment eects on biomass allocation and morphology. 4 Inhibition of pressurized ventilation resulted in decreased allocation to belowground weight and decreased rhizome penetration into the substrate in two of three experiments. Treatment also decreased growth rate, rhizome length and number of rhizomes when substrate had a high organic content. In the third experiment, growth clearly decreased in deep water, although inhibition of pressurized ventilation did not aect growth, biomass allocation or morphology at either of the water depths tested. 5 Decreased allocation to below-ground parts and decreased rhizome lengths may be adaptations to allow the oxygen concentration in roots and rhizomes to be maintained above a critical level when the oxygen supply is low. 6 Pressurized ventilation may improve the performance of P. australis but only under certain conditions (e.g. not when growth rate is low or the substrate has a high redox potential).