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col irli e F Ranong 85000, Thailand. Received 19 June 2001 and accepted in revised form 6 September 2001The diversity, abundance, biomass and community structure of crustacean and molluscan macrofauna were studied in the Ranong mangrove forest ecosystem on the Andaman Sea coast of southern Thailand. After a history of commercial exploitation the mangroves along the Klong Ngao tidal creek have been assigned conservation status within a new Ranong Biosphere Reserve established in 1997. Over the past 12 years, several areas of mangrove destroyed or degraded by wood harvesting, tin mining and aquaculture, have been rehabilitated on a pilot basis by planting monocultures of mangrove seedlings using four common local species (Rhizophora apiculata, R. mucronata, Bruguiera cyclindrica and Ceriops tagal). These plantation forests with different past management histories were compared with a natural, mixed, mature mangrove forest which has been conserved for about 40 years. Macrofauna were sampled within a 100 m2 vegetation quadrat in each study site. Crustaceans were sampled quantitatively by 315 min timed hand catches per site. Molluscs were sampled in 3 m2 quadrats positioned around three randomly selected trees in each vegetation quadrat. The lowest crustacean and molluscan diversity was recorded from the former tin mining site. The highest diversity was recorded fro