Diurnal and seasonal variations in carbon dioxide and methane fluxes between Sundarban biosphere and atmosphere were measured using micrometeorological method during 1998-2000. Study of the diurnal variation of micrometeorological conditions in the atmosphere was found to be necessary to determine the duration of neutral stability when flux estimation was reliable. Neutral stability of the atmosphere occurred in the limited micrometeorological conditions, when friction velocity ranged between 0.360 and 0.383 m s-1. The value of drag coefficient (1.62-20.6) × 103 obtained at variable wind speed could be deemed specific for this particular surface. 58.2% drop of carbon dioxide and 63.4% drop of methane in the atmosphere at 1 m height were observed during day time, between dawn and early evening. Diurnal variations in methane and carbon dioxide mixing ratios showed a positive correlation with Richardson's number (Ri). This environment acted as a net source for carbon dioxide and methane. The mixing ratios of methane were found to vary between 1.42 and 2.07 ppmv, and that of carbon dioxide, between 324.3 and 528.7 ppmv during the study period. The biosphere-atmosphere flux of carbon dioxide ranged between -3.29 and 34.4 mg m-2 s-1, and that of methane, between -4.53 and 8.88 μg m-2 s-1. The overall annual estimate of carbon dioxide and methane fluxes from this ecosystem to atmosphere were estimated to be 694 Tg yr-1 and 184 Gg yr-1, respectively. Considerable variations in mixing ratios of carbon dioxide and methane at the NE coast of Bay of Bengal were observed due to the seasonal variations of their fluxes from the biosphere to the atmosphere. The composition was inferred by fitting model prediction to measurements. © 2002 Elsevier Science Ltd. All rights reserved.