The microwave spectrum of 2-cycloheptene-1-one, an unsaturated cyclic ketone, has been studied in the regions 26.5-40 and 7.0-12.4 GHz. An analysis of the ground-state " a"-type transitions yielded the rotational constants (in MHz): A = 2997.27, B = 2049.24, C = 1399.76. The " a"-type transitions of an excited vibrational state were also assigned, giving A = 3000.51, B = 2046.65, C = 1398.88. The centrifugal distortion constants, DJ and DJK, were needed to fit the data adequately. A study of the Stark effect yielded the dipole moment components (in debye) μa = 3.63 ± 0.023 and μc = 0.882 ± 0.040. The μb component could not be determined from the Stark effect data. These data are used to discuss the molecular conformation of cycloheptene-1-one.