The Mg/Ca ratio in foraminiferal shells is widely used as a proxy for paleotemperatures. Nevertheless, it seems that the basic Mg content of foraminifera is determined by biological factors, as can be concluded from the large inter species and intrashell variability and the frequent deviations from inorganic behavior. This paper discusses three possible ways by which foraminifera can control or modify the Mg content in their shell: (1) involvement of organic matrix in the precipitation process that may alter the partition coefficient of Mg in biogenic calcite, (2) controlled conversion of transient amorphous phases to calcite, and (3) modification of the Mg concentration in the parent solution from which the crystals precipitate. The first two mechanisms are probably responsible for the precipitation of high-Mg calcite phases (whole shell or sublayers), while the third mechanism leads to the formation of low-Mg calcite phases. We propose a model adapted from epithelial cells that allows massive Mg2+ removal from the biomineralization site. This model is especially relevant to the planktonic and deep benthic low-Mg foraminifera that are frequently used for paleotemperature reconstructions. We discuss the possible biological roles of Mg in the shell in terms of the calcite polymorph conservation, the in vivo chemical stability of the shell, the functions of Mg as a stabilizer of transient phases and as a controlling agent of the precipitation process. Several temperature sensitive biological processes that may influence the Mg/Ca ratio of the shell are suggested and a model that combines biogenic and inorganic considerations is presented. The model uses Mg heterogeneity in the shell together with temperature response (biologic and inorganic) of biomineralization processes, to account for the deviation of planktonic foraminifera from inorganic calcite at equilibrium with seawater.