Dark-grown Euiglena gracilis vrar. bacillaris Pringsheim has been shown to contain no chlorophyll and is unable to carry out photosynthetic reactions. In addition, dark-grown cultures contain only level of 1earance of photosynthetic competence, and its complete absence in dark-grown cells, raises the question of the possible relationship between neoxanthin and photosynthetic processes. Epoxide carotenoids had been suggested as participants in both photosynthetic O, evolution (4, 18) and 02 transport (2), but little information was available until 2 recent experiments using H2 180 appeared (20, 22). Although both experiments rule out the dliepoxide violaxanthin as an intermediate 02 transporter during the Hill reaction in spinach chloroplasts, the evidence with respect to neoxanthin is less certain. Yamamoto et al. (22) analyzed neoxanthin under adverse instrumental conditions, while Shneour and Calvin (20) did not have a sufficient quantity of neoxanthin to analyze.Of the 2 epoxide carotenoids in E. gracilis, only neoxaanthin parallels the course of development of photosynthetic competence with respect to both the initial lag period and the leveling off at about 90 hours (21). It still remains to be demonstrated that antheraxantlhin is present in other etiolated forms of E. gracilis, such as those produced by heat, antibiotics or antihistaamine. In the drug-bleached and heat-bleached strains investigated by Goodwin and Gross (8), neoxanthin was lacking. It would therefore appear as if neoxanthin is closely related to the photosynthetic system of E. gracilis.Trollein and zeaxanthin both appear to increase with time, although the (lata for zeaxanthin show rather wide variations. The miiinor pigments cryptoxanthin, eclhinenone and euglenanone were not separatedl; the total amount also shows an approximate linear increase with time. Similar results are found with hydroxyechinenone.