The microwave spectrum of gaseous bromomethyl cyclopropane (CH2CH2CHCH2Br) is reported in the range 12 to 36 GHz. Lines of the 79Br and 81Br species of cis and gauche forms are assigned and partial r0-structures derived. The rotational constants in MHz are: gauche C3H5CH279Br, A = 11 469.285, B = 1 374.777, C = 1 295.394; gauche C3H5CH281Br, A = 11 400.100, B = 1 364.088, C = 1 283.952; cis C3H5CH279Br, A = 8 759.918, B = 1 597.413, C = 1 522.141; cis C3H5CH281Br, A = 8 716.552, B = 1 583.761, C = 1 509.017. © 1979.