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RubisCO, the CO2 fixing enzyme of the Calvin-Benson-Bassham (CBB) cycle, is responsible for the majority of carbon fixation on Earth. RubisCO fixes (CO2)-C-12 faster than (CO2)-C-13 resulting in C-13-depleted biomass, enabling the use of delta C-13 values to trace CBB activity in contemporary and ancient environments. Enzymatic fractionation is expressed as an epsilon value, and is routinely used in modelling, for example, the global carbon cycle and climate change, and for interpreting trophic interactions. Although values for spinach RubisCO (epsilon = similar to 29 parts per thousand) have routinely been used in such efforts, there are five different forms of RubisCO utilized by diverse photolithoautotrophs and chemolithoautotrophs and epsilon values, now known for four forms (IA, B, D and II), vary substantially with epsilon = 11 parts per thousand to 27 parts per thousand. Given the importance of epsilon values in delta C-13 evaluation, we measured enzymatic fractionation of the fifth form, form IC RubisCO, which is found widely in aquatic and terrestrial environments. Values were determined for two model organisms, the 'Proteobacteria' Ralstonia eutropha (epsilon = 19.0 parts per thousand) and Rhodobacter sphaeroides (epsilon = 22.4 parts per thousand). It is apparent from these measurements that all RubisCO forms measured to date discriminate less than commonly assumed based on spinach, and that enzyme epsilon values must be considered when interpreting and modelling variability of delta C-13 values in nature.