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The microwave rotational spectra of 3-fluorotoluene and its seven 13 C isotopic species have been recorded at natural abundance in the frequency range from 4 to 26 GHz using a pulsed molecular jet Fourier transform microwave spectrometer. The molecular structure comprising bond lengths and angles as well as parameters describing the methyl torsion were determined with high accuracy. Due to the very low torsional barrier of 17 cm 1 , the lowest torsional states of the vibrational ground state exhibited large splittings in the spectrum, which were modeled satisfactorily with a modified version of the program XIAM and the program aixPAM, two programs developed to treat the methyl internal rotation effects. They were also applied to refit the microwave data of 3,4-difluorotoluene to standard deviations close to measurement accuracy.