Our Streptavidin, B-phycoerythrin conjugate is an exceptionally bright, photostable probe with dual excitation maxima at 546 and 565 nm and an emission maximum at 575 nm, spectrally similar to Cy3. This conjugate has been reported to have quantum yields of up to 0.98 and extinction coefficients of 2.4 million cm^-1 M^-1, which is equivalent to 30 unquenched fluorescein and 100 unquenched rhodamine molecules of comparable wavelengths.