Heterotrophic CO₂ fixation is a significant yet underappreciated CO₂ flux in environmental carbon cycling. In contrast to photosynthesis and chemolithoautotrophy – the main recognized autotrophicCO₂ fixation pathways – the importance of heterotrophic CO ₂ fixation remains enigmatic. All heterotrophs – from microorganisms to humans – take up CO₂ and incorporate it into their biomass. Depending on the availability and quality of growth substrates, and drivers such as the CO₂ partial pressure, heterotrophic CO₂ fixation contributes at least 1 %–5 % and in the case of methanotrophs up to 50 % of the carbon biomass. Assuming a standing stock of global heterotrophic biomass of 47–85 Pg C, we roughlyestimate that up to 5 Pg C might be derived from heterotrophic CO ₂ fixation, and up to 12 Pg C yr⁻¹ originating from heterotrophic CO₂ fixation is funneled into the global annual heterotrophic production of 34–245 Pg C yr⁻¹. These first estimates on the importance of heterotrophic fixation of inorganic carbon indicate that this pathway should be incorporated in present and future carbon cycling budgets.