The biological synthesis of CaCO3 crystals of variable morphology by challenging non-calcareous microorganisms such as fungi with aqueous Ca2+ ions has been described. Many fungi are known to produce reasonable amounts of CO2 during growth. We show here that CO2 and characteristic proteins released from the fungi Fusarium oxysporum and Trichothecium sp. may be reacted with aqueous Ca2+ ions to produce truly biogenic CaCO3 crystals. While calcite crystals are obtained with both fungi, significant differences in the morphology of the crystals is observed, indicating that the proteins secreted by the fungi play a crucial role in directing the morphology of the calcite crystals. The action of specific proteins secreted by the microorganisms in directing calcite crystal morphology as well as the more complex issue of synergistic action of mixtures of proteins in directing crystal structure and morphology has been addressed.