Selective preservation of agglutinated foraminifera in the Great Marshes at Barnstable is caused by differences in test structure. The process may explain to a certain degree the differences in faunal composition, diversities and densities found between modem and fossil assemblages. A spring tide in combination with a storm or ice-scouting may transport foraminifera from lower habitats onto the high marsh. A thorough stratigraphic study is therefore essential to verify the absence of hiatuses in a selected core. Bioturbation and predation of living foraminifera do not alter the composition of assemblages that accumulate in subsurface marsh sediments in the Great Marshes. Agglutinated foraminifera with robust tests, such as Jadammina macrescens, Trochammina inflata, Haplophragmoides manilaensis, Balticammina pseudomacrescens, Tiphotrocha comprimata and A. mexicana, are well preserved in the sediment. Two of the three extant foraminiferal associations, characterised by species with robust tests, can be clearly recognised in the fossil assemblages. The third association poses some problems caused by selective preservation of Ammotium salsum and to a certain extent of Miliammina fusca. Deep infaunal habitat could explain why Arenoparrella mexicana is associated with different species in modem and fossil faunas. It is shown that it is feasible to apply the modem actuo-facies to the fossil assemblages in the Great Marshes.