Aqueous extracts of blue-green algae were fractionated by electrofocusing. In all algae investigated, fractions with isoelectric points at or near 4.6 showed photochromic behavior analogous to that of phytochrome, although they were sensitive to light of shorter wavelength. Three main types of photochromic pigments were found. Phycochrome a (in Tolypothrix distorta, Phormidium luridium, Nostoc muscorum 1453/12 and Anacystis nidulans) had 1 form absorbing maximally at about 590 nm (formed under red light) and 1 absorbing maximally at about 630 nm (formed under green light). Phycochrome b (in T. distorta) had 1 form absorbing maximally near 510 nm and 1 form absorbing maximally at 570 nm (formed in yellow-green and blue-green light, respectively). Phycochrome c (in N. muscorum A and probably in T. tenuis) had 1 form absorbing maximally at 650 nm (formed under green light) and 1 absorbing very weakly in the green region (formed under red light). The conversion of Phormidium phycochrome a from its red-absorbing form to its green-absorbing form caused the same spectral change as if an f-chromophore of phycocyanin were transformed into an s-chromophore. The quantum yield for this conversion was estimated to be 0.1, while the quantum yield for the reversion was estimated to be 0.4 on the assumption that the absorption coefficients were those of f- and s-chromophores. Phycochrome c was less light-sensitive than phycochromes a and b.