Cytochrome c(6), a plastocyanin functionally interchangeable electron carrier between the chlorophyll molecule P700 of photosystem I and cytochrome f from cytochrome b(6)f complex, has been isolated from the green alga Monoraphidium braunii and crystallized by the vapour-diffusion technique in sodium citrate. Crystals belong to space group R3, with cell dimensions a = b = 51.93 (5) and c = 80.5 (1) A (hexagonal axes), with one molecule per asymmetric unit. They diffract beyond 1.9 A under a Cu Kalpha rotating-anode source, with an anomalous signal that allows the positioning of the heme Fe atom in the unit cell.