The partial pressure of CO₂ (pCO₂) was measured at daily and weekly time scales in the waters surrounding mangrove forests in Papua New Guinea, the Bahamas and India. The pCO₂ values range from 380 to 4800 μatm. These data, together with previously published data, suggest that overall oversaturation of CO₂ with respect to atmospheric equilibrium in surface waters is a general feature of mangrove forests, though the entire ecosystems (sediment, water and vegetation) are probably sinks for atmospheric CO₂. The computed CO₂ fluxes converge to about +50 mmolC m⁻² day⁻¹. If this conservative value is extrapolated for worldwide mangrove ecosystems, the global emission of CO₂ to the atmosphere is about 50 10⁶ tC year⁻¹. Based on this tentative estimate, mangrove waters appear to be regionally a significant source of CO₂ to the atmosphere and should be more thoroughly investigated, especially at seasonal time scale.