Optimization of conditions for outdoor production of the nitrogen-fixing cyanobacterium Anabaena sp. ATCC 33047 has been pursued. In open ponds operated under semi-continuous regime biomass productivity values achieved ranged from 9 g (dry weight) m(-2) per day, in winter, to over 20 g m(-2) per day, in summer, provided that key operation parameters, including cell density, were optimized. Under these conditions the efficiency of solar energy conversion by the cells was fairly constant throughout the year, with photosynthetic efficiency values higher than 2%. The cyanobacterial biomass was rich in high-value phycobiliproteins, namely allophycocyanin and phycocyanin, for which open cultures of marine Anabaena represent a most interesting production system. The performance of Anabaena cultures operated under continuous regime in a closed tubular reactor has also been assessed outdoors, in winter. Biomass productivity values similar to those obtained in the ponds have been recorded for the closed system. Additionally, under these conditions, the cells excreted to the medium large amounts of a previously characterized exopolysaccharide, at production rates as high as 35 g m(-2) per day (1.4 g l(-1) per day). Properly operated closed cultures of this strain of Anabaena appear most suitable for outdoor mass production of valuable extracellular polysaccharides.