A new phycoerythrin, B-phycoerythrin, isolated from Porphyra naiadum, is described. The purification and crystallization, ultracentrifugation, electrophoretic and optical properties of this pigment are discussed. It has a major absorption peak at 545 mµ with a minor, and transient, one at 565 mµ, which tends to disappear on repeated crystallization. The molecular weight is apparently the same as that of R-phycoerythrin (ca. 290,000). Its iso-electric point is close to pH 4.5, and its mobility (toward the anode) at pH 5.0 is about 2 x 10-5 cm.2/sec./volt.