The rotational spectra of ethyl, n-propyl, and n-butylbenzene have been observed and analyzed using a high-resolution Fourier transform microwave spectrometer. Only one stable conformation was found for ethylbenzene, in agreement with previous results from Caminati [Mol. Phys. 74, 885 (1991)]. The high sensitivity of the technique allows observation of the rotational spectra of the six C-13 isotopes of ethylbenzene in natural abundance. The structural analysis indicates that the dihedral angle between the ethyl group and the ring plane is 88.7 degrees +/-0.9 degrees. Two conformers have been found for n-propylbenzene as predicted by ab initio calculations while four of the five theoretically predicted conformers [Dickinson , J. Chem. Soc., Faraday Trans. 93, 1467 (1997)] have been found for n-butylbenzene.