Rotational and fine-structure transitions in the SeH radical in the v= 0 level of its X 2? ground state have been detected by the technique of laser magnetic resonance at far- and mid-infrared wavelengths. These observations considerably extend previous work. The far-infrared spectra in particular were very strong and it was even possible to identify lines from 74SeH in natural abundance (0.9%). Some transitions in the mid-infrared spectrum were recorded as Lamb dips, despite the fact that they are magnetic dipole in character. These observations provide additional information on the proton and 77Se hyperfine splittings. All the rotational and fine-structure transitions have been fitted to a single model using the appropriate isotopic scaling factors. As a result, the molecular parameters for SeH in its ground state are more accurately determined. Partial information on the nuclear hyperfine structure has also been obtained.