Scanning tunneling microscopy (STM) imaging of four nucleic acid bases, adenine, guanine, thymine, and cytosine, has been performed on the surface of SrTiO3 single-crystal substrates. The images indicate that the bases are deposited as individual molecules in ultrahigh vacuum condition, and can be individually observed by STM. A comparison of the heights and lateral dimensions of the deposited species and also the bias dependence of the heights, enables differentiation between four nucleic acid bases in real space.