The rotationally resolved origin band in the 2 E-2(')<--X (2)A(1)(') electronic spectrum of cyclic B-3 has been observed by cavity ring down spectroscopy in the gas phase. The B-3 molecule was generated in a supersonic planar plasma containing decaborane (B10H14) and neon as a carrier gas. The rotational structure pattern is that of a cyclic molecule. It is analyzed assuming an equilateral triangle in both electronic states. The band origin is determined to be 21 853.52 cm(-1), and the bond lengths 1.603 77(106) Angstrom in the ground and 1.619 07(96) Angstrom in the excited electronic state are inferred from analysis of the rotational structure. (C) 2004 American Institute of Physics.