A 4.5 litre photobioreactor was developed and used to produce C-13-labelled beta-carotene by feeding (CO2)-C-13 to growing cells of the marine alga Dunaliella salina. Three runs were carried out using different feeding regimes producing beta-carotene containing 40, 56 and 76 Atom% C-13. Approximately 145 mg beta-carotene was recovered from each run. Incorporation of C-13 into beta-carotene ranged from 1.4 to 2.8%. The labelled beta-carotene produced would allow multiple tracer experiments for the study of human carotenoid metabolism.