The effect of ammonium phosphate on the growth, lipid content, and γ-linolenic acid accumulation was determined in the cyanobacterium Spirulina platensis. After 14 d on media containing 0.041 g N/L, γ-linolenic acid concentration of cultured cells increased up to 35.3 ± 0.13%, w/w. In treatments containing NaNO3, NH4NO3, and NH4Cl, γ-linolenic acid concentration increased up to 31.2 ± 0.23%, w/w. After the same period, lipid content in the dry biomass was 12.2 ± 0.03%, w/w, with (NH4)2HPO4 compared to about 14.1 ±.0.12%, w/w, in treatments with NaNO3. When (NH4)2HPO4 concentration in the medium was increased to 0.082 g N/L, 30.8 ± 0.28%, w/w, γ-linolenic acid had formed after 10 d and the lipid percentage in the dry cell mass was 16.7± 0.16%, w/w. However, in treatments with NaNO3, NH4NO3, or NH4CI, γ-linolenic acid concentration increased up to 30.6 ± 0.23%, w/w, and the lipid content was found to be 18.0 ± 0.17 to 18.9 ± 0.03%, w/w. These data showed that (NH4)2HPO4 is a suitable source of nitrogen for growth of S. platensis, with increased accumulation of γ-linolenic acid at lower N concentration. Copyright © 1999 by AOCS Press.