The microwave spectrum of ethyl cyanide has been studied in the region from 17–36 kMc. Both parallel and perpendicular transitions have been assigned. Rotational constants (Mc) for the ground vibrational state are a0=27 663.30, b0=4714.14, c0=4235.14, DJ=0.0035, DJK=—0.0496. From Stark effects the dipole moment is calculated to be μa=3.78 D, μb=1.38 D, μ=4.02 D. Hyperfine splittings lead to a value of —3.3 Mc for the quadrupole coupling of the N14 nucleus along the a axis. Rotational transitions of molecules in several excited vibrational states have also been measured. A Coriolis interaction between the torsional mode and the in-plane CCN bend has been found and is discussed. Splittings of transitions of molecules in the first excited torsional state show that the barrier hindering internal rotation of the methyl group is 3.05 kcal/mole.