SUMMARY During chloroplast development, the large increases in ribulose diphosphate carboxylase (RUDPCase) activity and cytochrome 552 concentration follow the pattern of chlorophyll synthesis, in that the formation of these two enzymes is inhibited by streptomycin (Sm) and by chloramphenicol (Cm) beyond 12 h of de- velopment. Neither enzyme can be detected in W,BUL, a mutant of Euglena in which chloroplasts and chloroplast DNA are undetectable. In contrast, the NADP-linked triose phosphate dehydrogenase (NADP-TPDase), another plastid- localized enzyme, increases in activity without the 12 h lag normally observed for chlorophyll synthesis; this increase in activity is not inhibited by Sm and Cm, but is inhibited by cycloheximide, an antibiotic which acts on 87 S cytoplasmic ribosomes. NADP-TPDase activity is present at the same level in W,BUL as in the dark-grown wild-type organisms. These data are interpreted to mean that NADP-TPDase is coded in the nuclear DNA, and is translated on 87s cytoplasmic ribosomes. The sensitivity of the increase in cytochrome 552 and RUDPCase activities to Sm and Cm indicates that they are translated, at least in part, on the 68 S ribosomes of the chloroplast. Thus, chloroplast differentiation in Euglena is dependent upon in- formation and synthetic machinery from both the plastid and the rest of the cell. Since total cellular protein does not change significantly during chloroplast development in resting cells, we conclude that protein turnover probably occurs.