Four emission bands, all belonging to the main thallium centre, are detected in a CsI:Tl crystal and their spectral, polarization and kinetic characteristics are studied at 1.7 to 320 K. Two weak ultraviolet bands (3.31 and 3.09 eV) are ascribed to electronic transitions from trigonal and tetragonal Jahn-Teller minima of the triplet relaxed excited state of Tl+. Two intense visible bands (2.55 and 2.25 eV) are assumed to arise from two different off-centre configurations of a self-trapped exciton perturbed by the Tl+ ion.