The foraminiferal test is one of the principal sources of CaCO3 in oceans. The principal habitats of Amphistegina include the Indo-Pacific islands and the Caribbean Sea. Although quite rare in the Aegean Sea Amphistegina lobifera is typical of various regions throughout the eastern Mediterranean and has also been reported from the middle and north Aegean Sea. But, on the south-western coast of Turkey it is found in extensive amounts, to such an extent that, it forms a 50 cm thick layer of sand and changes the benthic structure, as well as the ecology. Our results are based upon 12 samples from Üç Adalar (Antalya), and 13 samples from Beş Adalar (Antalya) collected from 4–30 m of depth. Each sample contains a high proportion of A. lobifera within the sediment. The exaggerated proportion of the A. lobifera test on the benthos points to certain conditions. Other benthic foraminiferans are not only scarce in genus and species number, but they are also found in very small populations. Even though there is no evidence of volcanic activity from west of the Gulf of Antalya, tectonic activity is present, resulting in formation of submarine springs which are located on the active faults. The presence of limestones with karstic characteristics in the Taurus Mountains combined with submarine springs cause salinity and temperature variations, as well as an increase in CaCO3 in the seawater. So alterations in the chemical and physical conditions of seawater may be responsible for this enormous expansion of the A. lobifera population.