The nucleation and crystal growth of calcium carbonate from supersaturated bicarbonate solutions in the presence of stearic acid monolayers has been studied by electron microscopy and electron and X-ray diffraction. Whereas rhombohedral crystals of calcite are randomly deposited in the absence of the monolayer, regiospecific crystallization of vaterite florets occurs at the organic surface. The florets develop initially as single crystalline biconvex discs oriented with respect to the negatively charged headgroups of the organic membrane such that the radial axis, which corresponds to the crystallographic  direction, is aligned perpendicular to the monolayer interface. Subsequent radial outgrowth results in the polycrystalline floret morphology. The degree of compression of the monolayer results in significant changes in nucleation. Solid-phase films are highly catalytic and generate vaterite discs with a wide size distribution. In contrast, nucleation on liquid-phase monolayers results in discs of more uniform particle diameter. Increased supersaturation and uncompressed films gave both calcite and vaterite polymorphs. Preferential orientation of the ab vaterite crystal face parallel to the monolayer surface is discussed in terms of the electrostatic accumulation of a Stern layer of Ca ions. Polymorph selectivity is determined by kinetic factors involving charge accumulation and possibly stereochemical recognition between the alignment of the stearic acid carboxylates and the orientation of carbonate anions in the vaterite unit cell. Geometric (epitaxial) matching at the interface is not a primary factor in regulating nucleation.