The laser-induced fluorescence excitation and the single vibronic level luminescence (SVL) spectra of 1,2,3,4-tetrahydronaphthalene (THN) or tetralin in a supersonic jet have been investigated. The origin for the S-1-S-0 transition has been found at 36 791 cm(-1). Both excitation and SVL spectra show a prominent progression of a low-frequency vibration which is assigned to the puckering "twist" motion involving C-2 and C-3 atoms. The vibronic features are consistent with a symmetrically twisted minimum energy configuration of the pseudo-six-membered cyclohexene ring. A rough estimate of the barrier to interconversion between different forms in the S-1 state has been obtained from the excitation spectrum which revealed an interaction between "twist" and "bend" vibrations.