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Isotopic substitution is a useful method to study the influence of nuclear motion on the kinetics of charge transport in semiconductors. However, in organic semiconductors, no observable isotope effect on field‐effect mobility has been reported. To understand the charge transport mechanism in rubrene, the benchmark organic semiconductor, crystals of fully isotopically substituted rubrene, 13C‐rubrene (13C42H28), are synthesized and characterized. Vapor‐grown 13C‐rubrene single crystals have the same crystal structure and quality as native rubrene crystals (i.e., rubrene with a natural abundance of carbon isotopes). The characteristic transport signatures of rubrene, including room temperature hole mobility over 10 cm2 V−1 s−1, intrinsic band‐like transport, and clear Hall behavior in the accumulation layer of air‐gap transistors, are also observed for 13C‐rubrene crystals. The field‐effect mobility distributions based on 74 rubrene and 13C‐rubrene devices, respectively, reveal that 13C isotopic substitution produces a 13% reduction in the hole mobility of rubrene. The origin of the negative isotope effect is linked to the redshift of vibrational frequencies after 13C‐substitution, as demonstrated by computer simulations based on the transient localization (dynamic disorder) scenario. Overall, the data and analysis provide an important benchmark for ongoing efforts to understand transport in ordered organic semiconductors. [ABSTRACT FROM AUTHOR]