The XUV spectra of thorium in three different regimes of optical thickness have been observed in a laser-produced plasma. An intense 5d-5f transition array, the profile and extent of which are strongly sensitive to the thorium ion density, dominates the spectrum. A number of lines belonging to the 5d¹⁰6s²-5d⁹6s²5f, 6p and 5d¹⁰-5d⁹5f, 6p transition arrays of Th XI and Th XIII, respectively, are identified with the aid of Hartree-Fock and Dirac-Fock calculations.