The microwave spectrum of 2-propenylphosphine (allylphosphine), H2C=CH-CH2-PH2, has been investigated in the 7-26.5 and 32-58.5 GHz spectral regions at about -40 degreesC. Three conformers were assigned. All three rotamers have an anti-clinal ("skew") conformation of the C=C-C-P chain of atoms. The orientation of the phosphino group is different in the three conformers denoted conformer I, II, and III, respectively. A distinction between the three forms can be made with reference to the lone pair (1p) of the phosphino group. In conformer I, the 1p-P-C-C link is anti-periplanar, whereas the 1p-P-C-C, chain is -syn-clinal ("+gauche") in conformer II and +syn-clinal ("-gauche") in conformer III. Conformer I is 1.4(3) kJ/mol more stable than II and 1.6(3) kJ/mol more stable than III. The dipole moment of conformer I was determined to be in units of 10(-30) Cm (note units): mu(a) = 2.544(14), mu(b) 0.503(25), mu(c), = 0.438(24), and mu(tot) = 2.630(20) [mu(tot) = 0.788(6) D]. The dipole moment of conformer II is mu(a) = 1.215(11), mu(b) = 3.14(12), mu(c) = 1.25(49), and mu(tot) = 3.59(31) [mu(tot) = 1.08(9) D], and the dipole moment of conformer III is mu(a) = 1.567(13), mu(b) = 2.090(21), mu(c) = 1.877(15), and mu(tot) = 3.59(31) [mu(tot) = 0.964(7) D]. Five vibrationally excited states were assigned for conformer 1, whereas two excited states were assigned for 11 as well as one for conformer III. Their vibrational frequencies were determined by relative intensity measurements. Quantum chemical calculations at the B3LYP and MP2 levels of theory reproduced the experimental rotational constants to within a few percent. Much poorer agreement is found for the components of the dipole moments (but the total dipole moment is well reproduced) as well as for the quartic centrifugal distortion constants, the error being mainly systematic in this latter case. The energy differences between the three conformers were calculated both at the Gaussian-2 and at the complete basis set levels of theory with results that are close to the experimental values.