The Sundarban mangrove forest (4264 km2) is about 3% of the total area of the world mangrove is interdependent on the coastal aquatic subecosystem (1781 km2). Ammonia concentration and flux measurements in the mangrove ecosystem and measurements in such tropical land-ocean boundary conditions are few and far between. A micrometeorological method was used to measure ammonia exchange over Sundarban mangrove forest and aquatic subecosystem. Results showed both emission and gaseous dry deposition in the forest area at a rate 1.79×103 and 2.26×103 kg N km-2 yr-1, respectively. Flux of ammonia in the aquatic subecosystem was found always from air to water with a rate of 775.7 kg N km-2 yr-1. The annual particulate dry and wet atmospherically deposited ammonia flux were calculated to be 69.0 and 21.5 kg N km-2 yr-1, respectively. Exchange of gaseous ammonia in the mangrove forest showed net deposition with a rate of 4.90×10 2 kg N km-2 yr-1. Annual imported flux from the proximate terrestrial source to this virgin forest was found to be 4.26×106 kg N yr-1 out of which 32.9% was deposited to the coastal water. Ammonia compensation point for mangrove vegetation was found to occur between 51 and 421 nmol m-3 and its variation was related to wind velocity, temperature, and humidity. © 2005 Elsevier Ltd. All rights reserved.