Three new bands of (\mathrm{CoH}) and five new bands of (\mathrm{CoD}) have been observed using laser excitation in the visible and near-infrared, following reaction of Co atoms with (\mathrm{H}{2}) or (\mathrm{D}{2}) under supersonic jet-cooled conditions. All the bands reported here, together with two reported previously by Varberg et al. (J. Mol. Spectrosc. 138, 630-637, 1989), are shown to belong to a single electronic transition with an origin near (12400 \mathrm{~cm}^{-1}), here designated (A^{\prime 3} \boldsymbol{\Phi}{4}-X^{3} \boldsymbol{\Phi}{4}). High-resolution spectra of the ((0), 0 ) bands of both (\mathrm{CoH}) and (\mathrm{CoD}) reveal extensive hyperfine structure associated with the ({ }^{59} \mathrm{Co}) nucleus, which shows that the electron configuration of the (A^{3} \Phi) state is ((7 \sigma)^{1}(3 d \delta)^{3}(3 d \pi)^{3}(8 \sigma)^{1}). Wavelength-resolved fluorescence from one of the CoD bands has given the (\Omega=4-3) spin-orbit interval for the (X^{3} \Phi) state as (729 \mathrm{~cm}^{-1}) and confirmed the presence of a low-lying (\Omega=3) electronic state (\sim 2444 \mathrm{~cm}^{-1}) above the ground state. C1 1995 Academic Press, Inc.