The red microalga Porphyridium cruentum was cultured semicontinuously at a nutrient concentration of 16 mmol N.l(-1), and seven rates of daily renewal of the culture volume (2%, 5%, 10%, 20%, 30%, 40% and 50%) in order to establish the optimal conditions for the production of the biotechnologically valuable compounds phycoerythrin, sulfated polysaccharide and polyunsaturated fatty acids. The concentration of phycoerythrin was directly controlled by the availability of nitrogen in the culture medium, increasing with renewal rate while nitrogen was limiting and decreasing under nitrogen sufficient conditions, that were achieved with renewal rates higher than 10%. The concentration of total soluble exopolysaccharide followed a parabolic distribution with increasing renewal rates similar to that of phycoerythrin. On the other hand, the percentage of sulfatation of the excreted polysaccharide was stimulated at highest renewal rates and therefore with increasing nutrient availability and effective light intensity in the cultures. Total polyunsaturated fatty acid percentage remained stable with renewal rate, but the percentage of eicosapentaenoic acid (20 : 5n-3) increased with increasing renewal rates from 3% to 27% at the expenses of linoleic(ls : 2n-6) and arachidonic (20 : 4n-6) acids. Therefore, different strategies of cultivation should be applied for the production of each of the valuable compounds in semicontinuous cultures of P. cruentum.