For the prolate asymmetric ( κ = -0.58) top molecule dimethylketene the centrifugal constants have been determined in terms of the ∆ constants by means of ∆ K- ≠ 0 rotational transitions. Two earlier assigned torsional excited states (01, 10) 1 and (01, 10) 2 of symmetry B2 and A2 of C2 v could be confirmed. A third fundamental vibration vr( b1), probably the inplane skeletal rock, has been found and analyzed in the vr = 1 and vr = 2 states. The variation of the centrifugal constants and of the inertial defects with the state of excitation could be explained by Coriolis coupling between the vr( b1) = 1 vibrational and vt( b2) = 1 torsional excited states. The Coriolis constant ζ rta and the torsional frequency ωt( b2) have been estimated to be ζ rta ≈ 0.24 and ωr( b1) - ωt( b2) ≈ 5.8 cm -1, respectively.