A new series of high-affinity cyanine dyes was tested for the visualization of the dynamics of single DNA molecules through a fluorescence microscope. In particular, YOYO-1 (1,1'-(4,4,7,7-tetramethyl-4,7- diazaundecamethylene)-bis-4-[3-methyl-2,3-dihydro-(benzo-1,3-oxazole)-2- methylidene]-quinolinium tetraiodide) forms a very stable, highly fluorescent complex with double-stranded DNA and dramatically improves the quality of the images. We have characterized the factors affecting the signal/background in the imaging of single DNA molecules by fluorescence microscopy and compared the results obtained using YOYO-1 with those obtained using standard fluorescent dyes like ethidium bromide or acridine orange.