The diffusion of Ge dimers along the substrate dimer rows of Ge͑001͒ has been investigated with scanning tunneling microscopy. The jump frequency of on-top Ge dimers along symmetric dimer rows at room temperature is found to be eight times higher than the diffusion along asymmetric dimer rows ͑0.36 s −1 versus 0.044 s −1 ͒. We ascribe this difference to limitations associated with the rocking motion that a dimer has to perform while diffusing along asymmetric dimer rows.