The fluorescence spectrum of 1,3,5-tri-tert-butylpentalene (TTBP), the stable derivative of antiaromatic pentalene, has been measured in solution at room and low temperature in condensed phase. A good correspondence between the first allowed absorption band and the fluorescence spectrum is observed. The fluorescence quantum yield of TTBP 2 x 10(-4) M in cyclohexane is estimated to be approximate to 22 x 10(-3) (lambda(ex) = 313 nm) at room temperature. At low temperature and exciting at 355 nm, the fluorescence spectrum of TTBP shows appreciable vibronic structure. The fluorescence data have been related to the electronic structure of the parent molecule. MCSCF/CAS calculations for the ground and lowest excited states of pentalene, up to S-4, have been performed in D-2h symmetry with 3-21G and 6-31G basis sets including polarization and diffuse orbitals. As a result of the strong interaction between the pairs of states S-0/S-1 and S-2/S-3, the lowest component of each pair distorts from D-2h to lower symmetry, giving rise to new equilibrium geometries. According to ab initio calculations, the observed fluorescence is assigned to upper stale, S-3-->S-0, emission.