The exciting possibility of biosynthesis of minerals of variable morphology and with polymorph selectivity by challenging microorganisms such as fungi and actinomycetes has been described. Many fungi and actinomycetes are known to produce reasonable amounts of CO2 during growth. We show here that CO2 and characteristic proteins released from an endophytic fungus, Verticillium sp. and an extremophilic actinomycete, Thermomonospora sp. may be reacted with aqueous Ca2+ and Ba2+ ions to produce truly biogenic CaCO3 and BaCO3 crystals. While extracellular synthesis of the highly unstable vaterite polymorph of CaCO3 in a spherical morphology was observed with the fungus, both extra- and intracellular formation of CaCO3 in the form of composites of flat plates and branched elongated flat plates, were observed with the actinomycete. Reaction of Ba2+ ions with Verticillium sp. and Thermomonospora sp. resulted in the extracellular synthesis of BaCO3 crystals of spherical and flat, plate-like morphologies respectively. The action of specific proteins secreted by the microorganisms in directing crystal structure and morphology has been addressed.