The pure rotational spectrum of BaSH ((X) over tilde (2)A') and that of its deuterium isotopomer were measured using millimeter direct absorption techniques in the range 305-365 GHz. This work is the first time barium hydrosulfide has been observed by any spectroscopic technique. These radicals were synthesized in a dc discharge by the reaction of barium vapor, produced in a Broida-type oven, and either H2S or D2S. Thirteen rotational transitions of BaSH were recorded, as well as five transitions of BaSD; for each transition, asymmetry components for K-a=0 through K-a=6 or 7 were typically measured. Fine structure splittings, which generally ranged from 52 to 62 MHz in magnitude, were also observed in every transition. These data unambiguously demonstrate that BaSH is a bent molecule with C-s symmetry, following the trend established in the lighter alkaline earth hydrosulfides. Perturbations were found in the pure rotational spectra, however, likely resulting from accidental degeneracies and strong vibration-rotation coupling. From these measurements, rotational and fine structure parameters were established for BaSH and BaSD. An r(0) analysis indicates that the Ba-S-H angle is 88degrees, while r(Ba-S)=2.807 Angstrom and r(S-H)=1.360 Angstrom. A comparison of spin-rotation parameters with other hydrosulfides suggests that this interaction is dominated by second-order spin-orbit contributions in BaSH, generated by rotational mixing of nearby excited electronic states. Calculation of the g-tensor values for the unpaired electron in this radical suggests an elongated distribution about the metal atom primarily along the (c) over cap axis, in contrast to MgSH and CaSH, where the major asymmetry lies along the (b) over cap axis. (C) 2003 American Institute of Physics.