We report on STM imaging of a polydiacetylene (the monomer is CH3(CH2)11‐C≡C‐C≡C‐(CH2)8‐COOH) on graphite. The monomer is topochemically polymerized on the graphite surface by exposure to ultraviolet light. These images represent, for different concentrations of material on the surface, individual molecules of the polymer, an ordered layer of polymer molecules and crystalline aggregates of the polymer.