We propose that Bahamian whitings, floating patches of lime mud, are in part the result of biological precipitation of calcium carbonate induced by picoplankton and cellular components. The cells may act as nucleation sites for crystallization from seawater supersaturated with CaCO3. Our model represents a new hypothesis concerning Bahamian whiting formation and is based on several lines of evidence. Biochemical data suggest that the macromolecules from carbonate suspended in Bahama Banks whitings are distinct from those found in the lime mud sediment producer Penicillus and from bottom sediment. Direct ultrastructural evidence indicates that mineralization occurs on the surfaces of picoplankton cells and degrading organic cellular components. The organic constituents include, but are not restricted to, whole and fragmented algal cells and composite structures ∼20-30 μm in diameter. The observed calcium carbonate crystals are distinct from skeletal debris. Because of the magnitude of the whiting phenomenon, calculations of carbonate sediment production budgets should include the potential contribution of calcium carbonate produced by epicefular precipitation. Knowledge of carbonate mud genesis is critical to interpretation of ancient occurrences of lime mud as well as to understanding ancient and global carbon cycles.