The interrelation of the copper protein plastocyanin, and a soluble c-type cytochrome, c-552, in photosynthetic electron transport has been studied in the genus Chlamydomonas. With C. reinhardtii the plastocyanin: cytochrome c-552 ratio could be changed from 300:1 less than 1:16 simply by omitting copper from the medium, without any other detectable change. Plastocyanin was indetectable in a second species, C. mundana, for which the cytochrome c-552 level was always very high. The properties of Levine's C. reinhardtii mutant lacking plastocyanin, ac-208, were studies and it was found that the photosynthetic capabilities of a suppressed phenotype and suppressed genotype could be explained by reference to the cytochrome c-552 levels. Both proteins were successfully used in reconstitution experiments with chloroplast fragments. Both showed very fast kinetics for reduction by purified Chlamydomonas cytochrome f, but the rate of electron transfer from one to the other was much slower. It is concluded that they constitute an interchangeable pair, and the rationale for this and possible analogies are both discussed.