Greenhouse gas (GHG) emissions of CH4 and CO2, resulting from decomposition of flooded organic matter from the hydroelectric reservoir of Petit Saut in the tropical rain forest of French Guiana have been monitored since reservoir impoundment in January 1994. This data set along with complementary data taken from older reservoirs in forested regions of the southern Ivory Coast provides an estimate of long-term GHG emission trends from a tropical reservoir. The trends are used to calculate the contribution of this reservoir to global warming on a 100 year timescale, assumed to be consistent with the life cycle of the reservoir. Calculations are based on the concept of global warming potential (GWP). Natural emission of greenhouse gases (CH4 and N2O) from soils of the reservoir before impoundment is estimated through field measurements and literature data. Then net GHG emissions from the reservoir on a 100 hundred year timescale (30 million tons of equivalent CO2, with an uncertainty range of 7--54 Mt CO2eq) are compared with predicted emissions from thermal power plants of equivalent power (115 MW). The final comparison takes into account the actual energy production of the dam power station at only 50% of the installed capacity. Emission from this reservoir, whose power density is low (0.315 MW km-2 flooded), would be similar to emissions from a gas power plant (33 Mt CO2eq) producing the same energy amount and less than emissions from other thermal alternatives, among which the most polluting are coal plants. Such a result, however, strongly depends on the choice of the integration time. ¿ 2001 American Geophysical Union