Early diagenetic ultrastructural alterations of benthic foraminifers of the genera Elphidium and Ophtalmina from the shallow water sediments of the Kiel Bight were investigated by scanning electron microscopy. Pure solution patterns were deduced from supplementary experiments.Several carbonate destroying processes can be specified by ultrastructural patterns of the shell surfaces. Based on these patterns three zones are established, each showing different mechanisms of shell fragmentation: 1) zone of abrasion, 2) zone of disintegration, 3) zone of corrosion. This zonation depends on the water depth and is caused primarily by water agitation and by undersaturation of the bottom water with respect to carbonate.