The rotational spectrum of the unstable HC3P molecule has been investigated in the millimeter-wave region. Five different isotopomers were studied, including also the three C-13-containing species which were detected in natural abundance. The obtained ground state rotational constants allowed us to evaluate the r(0) and r(s) molecular structures of phosphabutadiyne. CCSD(T) calculations were performed using the cc-pVQZ basis, and theoretically computed vibration-rotation coupling constants were combined with experimental ground state rotational constants to determine an accurate equilibrium structure.