The optimization of the operation, under the climatic conditions of southern Spain, of an experimental plant for β-carotene production by Dunaliella has been pursued. The effects of mixing, culture depth, cell density and dilution cycles on β-carotene and biomass productivity were studied under a semicontinuous culture regime in open tanks outdoors. Using 3 m²-surface containers, the highest productivity values, for both β-carotene and biomass, were recorded with a flow rate of 0.55 m s⁻¹; 10 cm depth; 0.7 − 0.9 × 10⁶cell ml⁻¹, population density; and dilution cycles of two days. An average annual productivity of 1.65 g (dry wt) m⁻² d⁻¹ was estimated for Dunaliella biomass, being that for β-carotene of about 0.1 g m⁻² d⁻¹. Under these optimized conditions, experiments have been carried out at the Cadiz Bay with 20 m²-surface tanks during a whole-year cycle. The results obtained have validated this location and the operating conditions established as being most appropriate for efficient mass production of β-carotene rich D. salina.