Abstract This work is focused on the influence of dilution rate (0.08 ≤ D ≤ 0.32 d−1) on the continuous cultivation and biomass composition of Arthrospira (Spirulina) platensis using three different concentrations of ammonium chloride (cNo = 1.0, 5.0 and 10 mol m−3) as nitrogen source. At cNo = 1.0 and 5.0 mol m−3 the biomass protein content was an increasing function of D, whereas, when using cNo = 10 mol m−3, the highest protein content (72.5%) was obtained at D = 0.12 d−1. An overall evaluation of the process showed that biomass protein content increased with the rate of nitrogen supply (D cNo) up to 72.5% at D cNo = 1.20 mol m−3 d−1. Biomass lipid content was an increasing function of D only when the nitrogen source was the limiting factor for the growth (D cNo ≤ 0.32 mol m−3 d−1), which occurred solely with cNo = 1.0 mol m−3. Under such conditions, A. platensis reduced its nitrogen reserve in the form of proteins, while maintaining almost unvaried its lipid content. The latter was affected only when the concentration of nitrogen was extremely low (cNo = 1.0 mol m−3). The most abundant fatty acids were the palmitic (45.8 ± 5.20%) and the γ-linolenic (20.1 ± 2.00%) ones. No significant alteration in the profiles either of saturated or unsaturated fatty acids was observed with cNo ≤ 5.0 mol m−3, prevailing those with 16 and 18 carbons.