Strains sensitive to tryptophan have been isolated in the green algae Chlamydomonas eugametos, an obligate autotroph, and C. reinhardtii, a heterotroph. In dark conditions, the C. reinhardtii mutant showed no growth inhibition in the presence of tryptophan and sodium acetate. In both species, neither tyrosine nor phenylalanine reversed tryptophan's inhibitory effect, but the sensitivity was promoted by the addition of riboflavin, a photosensitizer. The mutants exhibited a hypersensitivity to H 2O 2, a suspected photoproduct of tryptophan. The mutants were also found to be sensitive to cystine and methionine. Thus, it is strongly suggested that the sensitivity is due to the photoproduct(s) of tryptophan and presumably of cystine and methionine.