When grown photoautotrophically, Chlorella zofingiensis strain CCAP 211/14 accumulates a significant amount of valuable carotenoids, namely astaxanthin and lutein, of increasing demand for use as feed additives in fish and poultry farming, as colorants in food, and in health care products. Under standard batch-culture conditions, this microalgal strain exhibits high values of both growth rate (about 0.04 h(-1)) and standing cell population (over 10(11) cells l(-1), or 7 g dry weight l(-1)). Lutein, in a free (unesterified) form, was the prevalent carotenoid during early stages of cultivation (over 0.3 pg cell(-1), equal to 4 mg g(-1) dry weight, or 20 mg l(-1) culture), whereas esterified astaxanthin accumulated progressively, to reach a maximum (over 0.1 pg cell(-1), equal to 1.5 mg g(-1) dry weight, or 15 mg l(-1) culture) in the late stationary phase. A differential response of lutein and astaxanthin accumulation was also recorded with regard to the action of some environmental and nutritional factors. C. zofingiensis CCAP 211/14 represents a unique model system for analyzing the differential regulation of the levels of primary (lutein) and secondary (astaxanthin) carotenoids. Relevant also from the biotechnological viewpoint, this photosynthetic organism, with outstanding attributes for fast photosynthetic growth and carotenoid accumulation, might prove most valuable for its application to the mass production of either or both lutein and astaxanthin.