The properties of cyclopentadienylphosphine have been investigated by means of Stark-modulation microwave spectroscopy and quantum chemical calculations at the MP2/aug-cc-pVTZ, B3LYP/6-311++G(d,p), and G3 levels of theory. Spectra attributable to two rotamers denoted conformers I and II have been assigned. Conformer I has a symmetry plane (C, symmetry) consisting of the bisectors of the cyclopentadiene ring and of the phosphino group with the lone electron pair of phosphorus pointing toward the carbon ring. In conformer II, the phosphino group is rotated approximately 120 degrees out of this plane. Relative intensity measurements have been made, and it was found that conformer II is more stable than I by 1.3(4) kJ/mol. The preferred conformer represents a borderline case of intramolecular hydrogen bond stabilization. The experimental and MP2/ aug-cc-pVTZ rotational constants differ by several percent, which indicates that the aug-cc-pVTZ basis set is not large enough to be able to predict an accurate structure for the two conformers that are close to the equilibrium geometries. 5-Substituted 1,3-cyclopentadienyl derivatives may undergo circumambulatory rearrangements. However, there is no manifestation of this effect in the microwave spectrum of cyclopentadienylphosphine.