Coccolith-associated polysaccharides (CAPs) are thought to be a key part of the biomineralization process in coccolithophores, however their role is not fully understood. We have used two different systems that promote different polymorphs of calcium carbonate to show the effect of CAPs on nucleation and polymorph selection in vitro. Using a combination of time-resolved cryo-transmission electron microscopy (cryoTEM) and scanning electron microscopy (SEM), we examined the mechanisms of calcite nucleation and growth in the presence of the intracrystalline fraction containing CAPs extracted from coccoliths from Gephyrocapsa oceanica and Emiliania huxleyi, two closely related coccolithophore species. The CAPs extracted from G. oceanica were shown to promote calcite nucleation in vitro, even under conditions favouring the kinetic products of calcium carbonate, vaterite and aragonite. This was not the case with CAPs extracted from E. huxleyi, suggesting that the functional role of CAPs in vivo may be different between the two species. Additionally, high-resolution synchrotron powder X-ray diffraction (SXPD) revealed that the polysaccharide is located between grain boundaries of both calcite produced in the presence of the CAPs in vitro and biogenic calcite, rather than within the crystal lattice.