Arthrospira platensis was cultivated in tubular photobioreactor using different photosynthetic photon flux densities (PPFD) and protocols of (NH4)(2)SO4 fed-hatch supply. Results were evaluated by variance analysis selecting maximum cell concentration (X-m), cell productivity (P-x), nitrogen-to-cell conversion factor (Y-X/N) and biomass, protein and lipid contents as responses. At PPFD of 120 and 240 mu mol-photons/m(2) s, a parabolic profile of (NH4)(2)SO4 addition aiming at producing biomass with 7% nitrogen content ensured X-m values (14.1 and 12.2 g/L, respectively) comparable to those obtained with NaNO3. At PPFD of 240 mu mol-photons/m(2) s, P-x (1.69 g/Ld) was 36% higher, although the photosynthetic efficiency (3.0%) was less than one-half that at PPFD of 120 mu mol-photons/m(2) s. Biomass was shown to be constituted by about 35% proteins and 10% lipids, without any dependence on PPFD or kind of nitrogen source. These results highlight the possible use of (NH4)(2)SO4 as alternative, cheap nitrogen source for A. platensis cultivation in tubular photobioreactors. (C) 2010 American Institute of Chemical Engineers Biotechnol. Prog., 26: 1271-1277, 2010