Rotational and fine-structure transitions in the SeH radical in the v= 0 level of its X ²Π 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.