Ethynylarsine (2a) is synthesized by reaction of arsenic trichloride with ethynyltributylstannane followed by a chemoselective reduction of the formed ethynyldichloroarsine (1a). The base-induced rearrangement of compound 2a on a solid base in VGSR conditions leads to ethylidynearsine (3a) characterized by H-1 and C-13 NMR and by microwave spectroscopy. The rotational spectrum of 3a is recorded in the frequency range 90-470 GHz. Rotational and centrifugal distortion constants are determined and the quadrupole hyperfine structure is analyzed. The structure of CH3C drop As has been estimated from the rotational constants of eight different isotopic species and from ah initio calculations. Compound 3a exhibits a low stability at 0 degrees C in deuteriochloroform (tau(1/2) Ca. 1 h) or in the gaseous phase at a pressure of 10 Pa (tau(1/2) Ca. 30 min).