The electronic spectrum of the BiS molecule is described by means of relativistic configuration interaction (CI) calculations employing effective core potentials. Spin-orbit coupling causes the Omega = 3/2 components of the X (2)Pi and A (4)Pi Lambda-S states to undergo an avoided crossing which causes perturbations to occur in the observed X-2-X-1 band system beginning at upsilon' = 5. The present calculations are able to explain these results on a nearly quantitative basis. Spectroscopic constants are computed for nine BiS states up to 20 000 cm(-1) and generally good agreement is found between theoretical and available measured values. Four of these states have not yet been observed experimentally and thus the present results should aid in subsequent searches for them. In addition, radiative lifetimes and electric dipole moments have been calculated for each of the BiS states with the relativistic CI wave functions. Finally, these results have been compared in detail with the analogous spectral data for the isovalent BiO molecule. c 1999 American Institute of Physics. [S0021-9606(99)31022-9].