The first microscopic observation of a roughening transition on a crystal surface is reported. The surface involved is a so-called satellite face of a modulated ((CH3)N4)Zn2Cl4 crystal. The orientation of such a face is directly coupled to the length of the structural modulation wave vector q. At a change of q the stable satellite orientations change. Applying in situ optical microscopy we observed growth and etch features on the (1012) satellite face; at the first commensurate-commensurate phase transition the roughening features suggest a sudden decrease of the edge free energy to zero.