Dunaliella bardawil Ben-Amotz & Avron, but not most other Dunaliella species, has a unique property of being able to accumulate, in addition to glycerol, large amounts of β-carotene when cultivated under appropriate conditions. These include high light intensity, a high sodium chloride concentration, nitrate deficiency and extreme temperatures. Under conditions of maximal carotene accumulation D. bardawil contains at least 8% of its dry weight as β-carotene while D. salina grown under similar conditions contains only about 0.3%. Electron micrographs of D. bardawil grown under conditions of high β-carotene accumulation show many β-carotene containing globules located in the interthylakoid spaces of the chloroplast. The same algae grown under conditions where β-carotene does not accumulate, contain few to no β-carotene globules. The β-carotene-rich globules were released from the algae into an aqueous medium by a two-stage osmotic shock technique and further purified by centrifugal ion on 10% sucrose. The isolated purified globules were shown by electron microscopy to be free of significant contamination and composed of membrane-free osmiophilic droplets with an average diameter of 150 nm. Reversed phase high performance liquid chromatography of a total pigment extract of the cells revealed the presence of β-carotene as the major pigment, together with chlorophylls a and b, α-carotene and the xanthophylls lutein, neoxauthin and zeaxanthin. β-Carotene accounted for essentially all the pigment in the purified globules. Analysis of the algal and globule β-carotene fractions by HPLC showed that the β-carotene was composed of approximately equal amounts of all-trans β-carotene and of its 9-cis isomer. Intact D. bardawil cells contained on a dry weight basis about 30% glycerol, 30% protein, 18% lipid, 11% carbohydrate, 9%β-carotene and 1% chlorophyll. The β-carotene globules were composed of practically only neutral lipids, more than half of which was β-carotene. It is suggested that the β-carotene globules may serve to protect D. bardawil against injury by the high intensity irradiation to which this alga is usually exposed in nature.