Effects on methanogenesis and CH 4 emission of three water table positions, sediment compo-sition and presence or absence of the emergent macrophyte Phragmites australis (Cav.) Trin. ex Steud. were studied in outdoor experimental mesocosms. Water table position significantly affected methanogenesis and CH 4 emission, the rates being 40% and 60% lower, respectively, in vegetated organic sediments with a water table 22 cm below the surface as compared with vegetated inundated sediments. Due to the high water-holding capacity of organic sediments, rates of methanogenesis and CH 4 emission in organic sediments with a water table 8 cm below the sediment surface were only slightly, but not statistically significantly different from rates in inundated sediments. Sandy sediments with water tables 8 cm below the sediment surface had very low methanogenic activity as compared with organic sediments (1 versus 55 mmol m −2 per day). The presence of P. australis in inundated sediments attenuated the rate of methanogenesis by 27%, enhanced the proportion of CH 4 oxidised from 7% to 18%, and as a result attenuated CH 4 emission by 34%. In vegetated sediments CH 4 emission peaked at midday and was lowest during the night and early morning in concert with the diurnal changes in internal convective flow in the plants. Internal gas transport through the plants accounted for 62% of the CH 4 emitted from vegetated sediments while ebullition dominated in unvegetated sediment. ©1999 Elsevier Science B.V. All rights reserved.