A novel red fluorescent protein, eqFP611, from the sea anemone Entacmaea quadricolor has been cloned in Escherichia coli. With excitation and emission maxima at 559 and 611 nm, this protein shows the most red-shifted emission and the largest Stokes shift of all non-modified proteins in the green fluorescent protein (GFP) family. The protein fluoresces over a wide pH range (4-10) with high quantum yield (0.45). Its photophysical properties make eqFP611 an excellent marker protein for in vivo labeling in eukaryotic systems as was shown by expression in a mammalian cell culture. eqFP611 has been crystallized in space group P6(5)22, with unit-cell parameters a = b = 77.26, c = 329.49 A. The unit cell contains 12 asymmetric units, with two monomers in each. A molecular-replacement solution has been obtained using the 48.4% homologous red fluorescent protein from Discosoma coral (DsRed).