Phytoene synthase (PSY) catalyses the first step in the production of carotenoids, which has been described as a key regulatory step in the carotenoids biosynthetic pathway. PSY gene from Dunaliella salina was constitutively expressed in Chlamydomonas reinhardtii under the control of the RBCS2 and HSP70A promoters and targeted to the chloroplast by the RBCS2 transit peptide. DsPSY overexpression resulted in a stable increase in the corresponding PSY transcript level and in the content of carotenoids such as violaxanthin, lutein, and beta-carotene, reaching between 125 and 260% the levels in control untransformed cells. (C) 2011 American Institute of Chemical Engineers Biotechnol. Prog., 27: 54-60, 2011