Scanning tunneling microscopy (STM) has been used to investigate the adsorption of benzene on nominally flat Si(100)-(2X1) substrates. STM images show that benzene adsorbs on top of the dimer rows bonding to the two;Si-Si dimer dangling bonds. Bias-dependent imaging indicates that the highest occupied molecular orbital of adsorbed benzene lies approximately 1.2 eV below the top of the valence band and that the lowest unoccupied molecular orbital is at least 3.5 eV above the highest occupied molecular orbital. At higher coverages, the benzene molecules are adsorbed on every other dimer along the dimer row and on every other dimer across the dimer rows resulting in a local c(4X2) periodicity, in agreement with the saturation coverage of similar to 0.25 monolayer.