Cyanobutadiyne (cyanodiacetylene), HCCCCCN, is sufficiently stable at low pressures to permit its rotational spectrum to be studied by microwave spectroscopy. The spectrum consists of a series of R-branch transitions typical of a linear molecule. The transitions with J = 9 to 14 which lie between 26.5 and 40.0 GHz have been measured for the vibrational ground state. Transitions have also been detected in natural abundance for all possible singly substituted 13C and 15N isotopic species. Deuteriated cyanobutadiyne, DCCCCCN, has also been synthesized and its ground state spectrum recorded. These measurements have enabled a complete substitution structure to be derived for the first time for a polyacetylene: r8(HC a) = 1.0569 ± 0.001, r8(C aC b) = 1.2087 ± 0.001, r8(C bC c) = 1.3623 ± 0.003, r8(C cC d) = 1.2223 ± 0.004, r8(C dC e) = 1.3636 ± 0.003, r 8( Ce N) = 1.1606 ± 0.001 Å (10 -10m) . The spectroscopic parameters for the ground state are B0 = 1331.3313 ± 0.001 MHz and D0 = 0.0257 ± 0.002 KHz. The dipole moment, determined from the Stark effects of the J = 9 and 10 lines, is 4.33 ± 0.03 Debye.