The quantum yields of fluorescence ($Phi$$_f$) and trans $ o$ cis photoisomerization ($Phi$$_{t$ o$ c}$</TEX>), trans-4-(2-(9-anthryl)vinyl)pyridine, an aza analogue of 1-(9-anthryl)-2-phenylethylene, were measured in several solvents at room temperature. $Phi$$_f$ and $Phi$$_{t$ o$ c}$</TEX> are 0.38 and < 0.01 in hexane and 0.02 and 0.38 in acetonitrile, respectively. As solvent polarity decreases, $Phi$$_{t$ o$ c}$</TEX> strongly reduced, whereas $Phi$$_f$</TEX> strongly increased. A singlet mechanism of trans $ o$ cis photoisomerization is suggested since $Phi$$_{t$ o$ c}$</TEX> and $Phi$$_f$</TEX> change in opposite direction.