Seasonal and spatial variation of dissolved and atmospheric methane (CH4) was measured in the estuaries of the Sundarban mangrove ecosystem from January to December 2003. This unique mangrove forest ecosystem (9630 km2), a part of the Hooghly-Matla estuarine system (NE coast of Bay of Bengal) comprises about 3% of the total area of the world's mangroves. Dissolved methane concentrations in the mangrove dominated estuarine water (Muriganga, Saptamukhi and Thakuran) were observed between 11.0 and 129.0 nmol L- 1 with emission rates between 1.97 and 134.6 μmol m- 2 d- 1. Methane emission showed maximum rate during post-monsoon when phytoplankton reached blooming proportion. During post-monsoon materials transported from the mangrove forest could stimulate phytoplankton bloom when transparency of water column was increased and river input of nutrient was low. However, dissolved methane concentrations in the Hooghly estuary, the main river water flushing channel showed lower values between 10.3 and 59.25 nmol L- 1 with emission rates between 0.88 and 148.6 μmol m- 2 d- 1. Methane concentrations in the salinity gradient zone of the Hooghly estuary did not show any decreasing trend with increased salinity due to its lateral transport from the mangrove forest situated at the lower stretch of the estuary. Dissolved methane concentration in the pore water collected from the virgin mangrove forest showed a maximum concentration of 5769 nmol L- 1 which was considerably lower compared to that of coastal salt marsh, tidal fresh water wetland. This indicated intense oxidation or out competition of methanogenic bacteria by sulfate and nitrate reducing bacteria. Methane emission from the adjacent forest ecosystem (18.36 mmol m- 2 d- 1) was higher compared to that in the mangrove water. Considering methane emission rates from the aquatic and forest ecosystems, an area average rate of 47.28 × 105 mol km- 2 yr- 1 for Sundarban mangrove environment was obtained. Extrapolating over the global area covered by mangrove forest with area average soil methane emission rate without considering its emission from water could yield inaccurate estimation. © 2006 Elsevier B.V. All rights reserved.