The microwave spectrum of bromoacetylene has been investigated in the frequency range from 7 GHz to 35 GHz. From Stark effect measurements the dipole moment has been determined as μ = 0.23 + 0.01 D, and the restructure has been derived in four independent ways from twelve isotopic species: C - H = 1.0553 Å, C ≡ C = 1.2038 Å, C - Br = 1.7913 Å (internal consistency better than ± 0.0003 Å). Quadrupole coupling constants have been determined for eleven isotopic forms and are e q Q = 541.47 MHz and 648.00 MHz for HCCBr 81 and HCCBr 79 , respectively. Rotation spectra have also been observed for excited states of the three lowest normal modes and for several isotopic forms. For the Br 81 (Br 79 ) species the rotation-vibration coefficients are α 5 = -10.98 (-11.02) MHz, α 4 = -1.57 (- 1.61) MHz and α 3 = + 12.88 (+ 13.40) MHz. For the bending vibrations, ν 5 and ν 4 , l-type doubling constants are obtained as ql 5 = 4.14 (4.17) MHz and ql 4 = 2.6 MHz. Analysis of the Fermi resonance between the first excited state of ν 3 and the l = 0 component of the second excited state of ν 5 gives the mixing ratio of these two states as a/b = 1.45 (1.51) and the interaction energy as W 3,5 = 1.313 (1.181) δ 3,5 for the Br 81 (Br 79 ) species. With an approximate value of δ 3,5 ≌ 25 cm -1 , the cubic force constant is obtained as κ 3,55 ≌ 44 cm -1 . The results are discussed in relation to the molecular properties of other halogen acetylenes and halogen cyanides.