Rotational transitions for methylferrocene were measured, in the 4-12 GHz range, using a Flygare-Balle type, pulsed-beam Fourier transform spectrometer. Mono-substituted ferrocenes are nearprolate asymmetric tops with a and b dipole moment components, providing numerous possible transitions in this frequency range. Eighteen rotational constants were calculated from the data for six isotopomers. 59 transitions were measured for the normal isotopomer, and much smaller data sets were obtained for the Fe-54, and four C-13 isotopomers. Despite the small data sets for C-13 and Fe-54 transitions, good fits were obtained with small standard deviations ranging from 2 to 5 kHz. The eighteen A, B, and C rotational constants were used to determine the structural parameters for methylferrocene. Measured rotational constants for the normal isotopomer are: A=1592.6050(6), B=957.2565(4), and C=825.9892(4) MHz. The values for the averaged structural parameters are: r(1)(Fe-C)=2.050(7) A, r(2)(C-C)(cyclopentadienyl ligands)=1.433(7) A and r(3)(C-CH3)=1.52(2) A (for the methyl cyclopentadienyl ligand). Small splittings (20-30 kHz) were observed for some transitions, and these could be due to proton hyperfine structure or small internal rotation splittings. The structural parameters for this complex were calculated using density functional theory (DFT) methods, and the results are in very good agreement with the measured values. Results are compared with earlier measurements on substituted ferrocenes. Distortions of the substituted cyclopentadienyl ligand are significantly smaller than those observed for chloroferrocene.