The microwave spectrum of cyclopropylethynylphosphine, C3H5C≡CPH2, has been investigated in the 26-120 GHz spectral region. The spectrum is dominated by very rich and complex a-type R-branch pile-ups. There must be insignificant steric interaction between the phosphino group and the cyclopropyl ring due to the long distance between these two groups. However, the phosphino group does not undergo free or nearly free internal rotation. Instead, the spectra of two distinct conformers were assigned. Both these two forms have CS symmetry. The symmetry plane bisects the cyclopropyl ring and the phosphino group in both conformers, and the lone electron pair of the phosphino group points in opposite directions in the two rotamers. The energy difference between the two forms was detd. to be 1.9(6) kJ/mol. A simple model that takes into consideration the interaction of the lone electron pair of the phosphino group with the π-electrons of the ethynyl group and the Walsh electrons of the cyclopropyl ring is able to give a qual. explanation of the observation of two conformers and the nonexistence of free rotation of the phosphino group. The MW work was augmented by quantum chem. calcns. using second-order Moller-Plesset perturbation and coupled cluster theory with results that are in good agreement with the expts. [on SciFinder(R)]