MB 660R is a bright and photostable far-red dye that emits fluorescence at about 685 nm in the borderline spectral region between far-red and near-IR. Although the absorption maximum is at around 665 nm, this dye can be sufficiently excited by the 633 or 635 nm laser. MB 660R dye is water soluble and pH-insensitive from pH 4 to pH 10. MB 660R is a rhodamine-based dye, and like rhodamine dyes in general, it is exceptionally photostable (Figure 1). The superior photostability and excellent brightness of MB 660R make the dye an ideal choice for confocal microscopy and other demanding applications.