We have developed a procedure for the isolation of Chlamydomonas reinhardtii mutants defective in light-dependent protochlorophyllide reduction (photoconversion), a key step in the biosynthesis of chlorophyll. Mutants were isolated by mutagenizing y-l-4, a temperature-sensitive yellow mutant blocked in the alternative light-independent protochlorophyllide reduction pathway, and screening for colonies which failed to green in the light at the restrictive temperature. Seven mutants were isolated which fail to photoconvert protochlorophyllide in photoconversion tests. All seven mutants have a single mutation at the pc-1 locus responsible for the defect in photoconversion. pc-1 maps close to y-5 on nuclear linkage group I. The pc-1 mutation is not itself temperature-sensitive because it blocks photoconversion at the permissive temperature when combined with the non-conditional yellow mutations y-5 and y-7. Cells containing the pc-1 mutation alone synthesize about 52% and 36% of the wildtype chlorophyll level in the dark and light, respectively, demonstrating that the light-independent protochlorophyllide reduction pathway in C. reinhardtii operates in the light.