Sulfite reductase (SiR) is an important enzyme catalyzing the reduction of sulfite to sulfide during sulfur assimilation in plants. This enzyme is locali-zed in plastids, including chloroplasts, and uses ferredoxin as an electron donor. Ferredoxin-dependent SiR has been found in isolated chloroplast nucleoids, but its localization in vivo or in intact plastids has not been examined.