The synthesis of the novel compound cyclopropylethylidynephosphine (C3H5CH2C=P) and its microwave spectrum are reported together with quantum chemical calculations. The spectrum, which reveals the existence of two conformers, has been recorded in the 38-109 GHz spectral range at room temperature. The H-C-CH2-C chain of atoms is synclinal in one rotamer denoted sc, and antiperiplanar in the second conformer called ap. The spectra of the ground vibrational state and two vibrationally excited states were assigned for each rotamer. The vibrational frequencies of these excited states were determined by relative intensity measurements. Relative intensity measurements were also conducted to determine the energy difference between ap and sc. The latter conformer was found to be the lower-energy form and Eap - Esc was determined to be 0.9(4) kJ/mol. The microwave study has been augmented by quantum chemical calculations at the CCSD/cc-pVQZ and MP2/cc-pVTZ levels of theory. The CCSD predictions were generally in good agreement with experiment, while somewhat mixed results were obtained in the MP2 calculations.