Abstract Light-induced absorbance changes of cytochrome b 559 and C 550 in chloroplasts indicate that noncyclic electron transport from water to ferredoxin (Fd)-NADP + is carried out solely by System II and includes not one but two photoreactions (IIa and IIb) that proceed effectively only in short-wavelength light. (C 550 is a new chloroplast component identified by spectral evidence and distinct from cytochromes.) The evidence suggests that the two short-wavelength light reactions operate in series, being joined by a System II chain of electron carriers that includes (but is not limited to) C 550, cytochrome b 559, and plastocyanin (PC). H 2O → IIb hv → C 550 → cyt. b 559 → PC → IIa hv → Fd → NADP + Photoreaction IIb involves an electron transfer from water to C 550 that does not require plastocyanin and is the first known System II photoreaction resistant to inhibition by 3-(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU) and o-phenanthroline. Cytochrome b 559 is reduced by C 550 in a reaction that is readily inhibited by DCMU or o-phenanthroline. Thus, the site of DCMU (and o-phenanthroline) inhibition of System II appears to lie between C 550 and cytochrome b 559. Photoreaction IIa involves an electron transfer from cytochrome b 559 and plastocyanin to ferredoxin-NADP +.