Four core samples were collected from the vicinity of Alibey and Maden islands on the northwestern of Ayvalık. 91 sediment samples were obtained from the cores and were analyzed for their recent foraminifer content. High abundance of individuals with morphological abnormalities were observed in benthic foraminifer species such as, Peneroplis pertusus, P. planatus, Lobatula lobatula, Ammonia compacta, A. parkinsoniana, Challengerella bradyi, Elphidium complanatum, E. crispum. The introduction and deposition of heavy metal with terrestrial origin in marine biota result in an environmental risk for aquatic life. Environmental pollution became common mainly in the second half of 20th century, because of dumping of materials, such as manganese, lead and iron to terrestrial and marine environments. Also managing the ore beds led to significant morphological changes in foraminiferal assemblages. The abnormal test development observed in foraminifers and presented in this study are suggested to be the result of biological effects of the heavy metals that reached marine environment, and this phenomenon can be used as a practical indicator to determine the natural environmental pollution. The aim of this study is to find out the possible reasons of these morphological abnormalities, and compare with other factors which are known to cause similar deformations.