High density CO2 fluid inclusion was found in one zircon crystal separated from commercial gravel-sand supplied from sand-pit at Hulín, where the Quaternary fluvial sediments are exploited. The prismatic, slightly rounded 19x10 μm large fragment of a zircon contained single elongated 6 μm long fluid inclusion which appears to be monophase at room temperature. The microthermometric measurements revealed presence of almost pure CO2 (melting temperature as low as -56.9 °C) that homogenized to liquid phase at -49.2 °C. The corresponding density for pure CO2 system is 1.152 g/cm3. Yet, fluid inclusions with similar characteristics are not known from the possible source area, from which the clastic material could have been derived. Theoretically, the dense CO2 fluids could be bound to high-grade metamorphic rocks (amphibolite facies or higher) or to low-grade metamorphic rocks in areas of low geothermal gradient.