Binding of the dye TO-PRO-3 iodide to alternating GC and AT oligomers of DNA is investigated by absorption and hole burning spectroscopies. This dye is one of a series extensively used for nucleic acid staining. In addition to narrow laser induced holes, the dye/oligomer complexes also exhibit white light induced hole burning. These holes are indicative of a dye/DNA binding mode in addition to the previously identified externally bound and intercalative modes. Possible structures of this mode are considered.